Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 2 Apr 2024 20:43:25 +0200 | Subject | Re: [PATCH v4 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code | From | David Hildenbrand <> |
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On 02.04.24 19:57, Peter Xu wrote: > On Tue, Apr 02, 2024 at 06:39:31PM +0200, David Hildenbrand wrote: >> On 02.04.24 18:20, Peter Xu wrote: >>> On Tue, Apr 02, 2024 at 05:26:28PM +0200, David Hildenbrand wrote: >>>> On 02.04.24 16:48, Ryan Roberts wrote: >>>>> Hi Peter, >>> >>> Hey, Ryan, >>> >>> Thanks for the report! >>> >>>>> >>>>> On 27/03/2024 15:23, peterx@redhat.com wrote: >>>>>> From: Peter Xu <peterx@redhat.com> >>>>>> >>>>>> Now follow_page() is ready to handle hugetlb pages in whatever form, and >>>>>> over all architectures. Switch to the generic code path. >>>>>> >>>>>> Time to retire hugetlb_follow_page_mask(), following the previous >>>>>> retirement of follow_hugetlb_page() in 4849807114b8. >>>>>> >>>>>> There may be a slight difference of how the loops run when processing slow >>>>>> GUP over a large hugetlb range on cont_pte/cont_pmd supported archs: each >>>>>> loop of __get_user_pages() will resolve one pgtable entry with the patch >>>>>> applied, rather than relying on the size of hugetlb hstate, the latter may >>>>>> cover multiple entries in one loop. >>>>>> >>>>>> A quick performance test on an aarch64 VM on M1 chip shows 15% degrade over >>>>>> a tight loop of slow gup after the path switched. That shouldn't be a >>>>>> problem because slow-gup should not be a hot path for GUP in general: when >>>>>> page is commonly present, fast-gup will already succeed, while when the >>>>>> page is indeed missing and require a follow up page fault, the slow gup >>>>>> degrade will probably buried in the fault paths anyway. It also explains >>>>>> why slow gup for THP used to be very slow before 57edfcfd3419 ("mm/gup: >>>>>> accelerate thp gup even for "pages != NULL"") lands, the latter not part of >>>>>> a performance analysis but a side benefit. If the performance will be a >>>>>> concern, we can consider handle CONT_PTE in follow_page(). >>>>>> >>>>>> Before that is justified to be necessary, keep everything clean and simple. >>>>>> >>>>>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> >>>>>> Signed-off-by: Peter Xu <peterx@redhat.com> >>>>> >>>>> Afraid I'm seeing an oops when running gup_longterm test on arm64 with current mm-unstable. Git bisect blames this patch. The oops reproduces for me every time on 2 different machines: >>>>> >>>>> >>>>> [ 9.340416] kernel BUG at mm/gup.c:778! >>>>> [ 9.340746] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP >>>>> [ 9.341199] Modules linked in: >>>>> [ 9.341481] CPU: 1 PID: 1159 Comm: gup_longterm Not tainted 6.9.0-rc2-00210-g910ff1a347e4 #11 >>>>> [ 9.342232] Hardware name: linux,dummy-virt (DT) >>>>> [ 9.342647] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) >>>>> [ 9.343195] pc : follow_page_mask+0x4d4/0x880 >>>>> [ 9.343580] lr : follow_page_mask+0x4d4/0x880 >>>>> [ 9.344018] sp : ffff8000898b3aa0 >>>>> [ 9.344345] x29: ffff8000898b3aa0 x28: fffffdffc53973e8 x27: 00003c0005d08000 >>>>> [ 9.345028] x26: ffff00014e5cfd08 x25: ffffd3513a40c000 x24: fffffdffc5d08000 >>>>> [ 9.345682] x23: ffffc1ffc0000000 x22: 0000000000080101 x21: ffff8000898b3ba8 >>>>> [ 9.346337] x20: 0000fffff4200000 x19: ffff00014e52d508 x18: 0000000000000010 >>>>> [ 9.347005] x17: 5f656e6f7a5f7369 x16: 2120262620296567 x15: 6170286461654865 >>>>> [ 9.347713] x14: 6761502128454741 x13: 2929656761702865 x12: 6761705f65636976 >>>>> [ 9.348371] x11: 65645f656e6f7a5f x10: ffffd3513b31d6e0 x9 : ffffd3513852f090 >>>>> [ 9.349062] x8 : 00000000ffffefff x7 : ffffd3513b31d6e0 x6 : 0000000000000000 >>>>> [ 9.349753] x5 : ffff00017ff98cc8 x4 : 0000000000000fff x3 : 0000000000000000 >>>>> [ 9.350397] x2 : 0000000000000000 x1 : ffff000190e8b480 x0 : 0000000000000052 >>>>> [ 9.351097] Call trace: >>>>> [ 9.351312] follow_page_mask+0x4d4/0x880 >>>>> [ 9.351700] __get_user_pages+0xf4/0x3e8 >>>>> [ 9.352089] __gup_longterm_locked+0x204/0xa70 >>>>> [ 9.352516] pin_user_pages+0x88/0xc0 >>>>> [ 9.352873] gup_test_ioctl+0x860/0xc40 >>>>> [ 9.353249] __arm64_sys_ioctl+0xb0/0x100 >>>>> [ 9.353648] invoke_syscall+0x50/0x128 >>>>> [ 9.354022] el0_svc_common.constprop.0+0x48/0xf8 >>>>> [ 9.354488] do_el0_svc+0x28/0x40 >>>>> [ 9.354822] el0_svc+0x34/0xe0 >>>>> [ 9.355128] el0t_64_sync_handler+0x13c/0x158 >>>>> [ 9.355489] el0t_64_sync+0x190/0x198 >>>>> [ 9.355793] Code: aa1803e0 d000d8e1 91220021 97fff560 (d4210000) >>>>> [ 9.356280] ---[ end trace 0000000000000000 ]--- >>>>> [ 9.356651] note: gup_longterm[1159] exited with irqs disabled >>>>> [ 9.357141] note: gup_longterm[1159] exited with preempt_count 2 >>>>> [ 9.358033] ------------[ cut here ]------------ >>>>> [ 9.358800] WARNING: CPU: 1 PID: 0 at kernel/context_tracking.c:128 ct_kernel_exit.constprop.0+0x108/0x120 >>>>> [ 9.360157] Modules linked in: >>>>> [ 9.360541] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G D 6.9.0-rc2-00210-g910ff1a347e4 #11 >>>>> [ 9.361626] Hardware name: linux,dummy-virt (DT) >>>>> [ 9.362087] pstate: 204003c5 (nzCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--) >>>>> [ 9.362758] pc : ct_kernel_exit.constprop.0+0x108/0x120 >>>>> [ 9.363306] lr : ct_idle_enter+0x10/0x20 >>>>> [ 9.363845] sp : ffff8000801abdc0 >>>>> [ 9.364222] x29: ffff8000801abdc0 x28: 0000000000000000 x27: 0000000000000000 >>>>> [ 9.364961] x26: 0000000000000000 x25: ffff00014149d780 x24: 0000000000000000 >>>>> [ 9.365557] x23: 0000000000000000 x22: ffffd3513b299d48 x21: ffffd3513a785730 >>>>> [ 9.366239] x20: ffffd3513b299c28 x19: ffff00017ffa7da0 x18: 0000fffff5ffffff >>>>> [ 9.366869] x17: 0000000000000000 x16: 1fffe0002a21a8c1 x15: 0000000000000000 >>>>> [ 9.367524] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000002 >>>>> [ 9.368207] x11: 0000000000000001 x10: 0000000000000ad0 x9 : ffffd35138589230 >>>>> [ 9.369123] x8 : ffff00014149e2b0 x7 : 0000000000000000 x6 : 000000000f8c0fb2 >>>>> [ 9.370403] x5 : 4000000000000002 x4 : ffff2cb045825000 x3 : ffff8000801abdc0 >>>>> [ 9.371170] x2 : ffffd3513a782da0 x1 : 4000000000000000 x0 : ffffd3513a782da0 >>>>> [ 9.372279] Call trace: >>>>> [ 9.372519] ct_kernel_exit.constprop.0+0x108/0x120 >>>>> [ 9.373216] ct_idle_enter+0x10/0x20 >>>>> [ 9.373562] default_idle_call+0x3c/0x160 >>>>> [ 9.374055] do_idle+0x21c/0x280 >>>>> [ 9.374394] cpu_startup_entry+0x3c/0x50 >>>>> [ 9.374797] secondary_start_kernel+0x140/0x168 >>>>> [ 9.375220] __secondary_switched+0xb8/0xc0 >>>>> [ 9.375875] ---[ end trace 0000000000000000 ]--- >>>>> >>>>> >>>>> The oops trigger is at mm/gup.c:778: >>>>> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page); >>>>> >>>>> >>>>> This is the output of gup_longterm (last output is just before oops): >>>>> >>>>> # [INFO] detected hugetlb page size: 2048 KiB >>>>> # [INFO] detected hugetlb page size: 32768 KiB >>>>> # [INFO] detected hugetlb page size: 64 KiB >>>>> # [INFO] detected hugetlb page size: 1048576 KiB >>>>> TAP version 13 >>>>> 1..70 >>>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd >>>>> ok 1 Should have worked >>>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with tmpfile >>>>> ok 2 Should have failed >>>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile >>>>> ok 3 Should have failed >>>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB) >>>>> ok 4 Should have worked >>>>> # [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (32768 kB) >>>>> >>>>> >>>>> So 2M passed ok, and its failing for 32M, which is cont-pmd. I'm guessing you're trying to iterate 2M into a cont-pmd folio and ending up with an unexpected tail page? >>>> >>>> I assume we find the expected tail page, it's just that the check >>>> >>>> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page); >>>> >>>> Doesn't make sense with hugetlb folios. We might have a tail page mapped in >>>> a cont-pmd entry. As soon as we call follow_huge_pmd() on "not the first >>>> cont-pmd entry", we trigger this check. >>>> >>>> Likely this sanity check must also allow for hugetlb folios. Or we should >>>> just remove it completely. >>> >>> Right, IMHO it'll be easier we remove it, actually I see there's one more >>> at the end, so I think we need to remove both. >>> >>>> >>>> In the past, we wanted to make sure that we never get tail pages of THP from >>>> PMD entries, because something would currently be broken (we don't support >>>> THP > PMD). >>> >>> There's probably one more thing we need to do, on allowing >>> PageAnonExclusive() to work with hugetlb tails. Even if we remove the >>> warnings and if I read the code right, we can BUG_ON again on checking tail >>> pages over anon-exclusive for PageHuge. >>> >>> So I assume to fix it completely, we may need two changes: Patch 1 to >>> prepare PageAnonExclusive() to work on hugetlb tails, then patch 2 to be >>> squashed into the patch "mm/gup: handle huge pmd for follow_pmd_mask()". >>> Note: not this patch to fixup, as this patch only does the "switchover" to >>> the new path, the culprit should be the other patch.. >>> >>> I have them attached below first, before I'll also go and see whether I can >>> run some arm tests later today or tomorrow. David, any comments from >>> anon-exclusive side? >> >> I added the PageAnonExclusive checks for hugetlb back then, because calling >> it on a tail page indicated real trouble for hugetlb. >> >> Well, and I didn't want to have runtime-hugetlb checks in PageAnonExclusive >> code called on certainly-not-hugetlb code paths. >> >> Personally, I'd fixup the problematic callsite where we know nothing nasty >> is happening (like we did for gup_must_unshare(), because we don't expect >> hugetlb tail pages from arbitrary other code). >> >> But as I'm getting closer to a folio_test_anon_exclusive() implementation as >> we speak (closer, but not done :) ... ), where I'd remove any such hugetlb >> special handling, I don't particularly care how we handle GUP here in the >> meantime. > > That's what I was looking for and found missing just now, when I wanted to > allow follow_huge_pmd() pass page / folio (which will be the head then) > properly into different checks. I think that patch 1 is the simplest I can > come up with that works mostly like what you said before a follow up > cleanup on top if possible. It mostly pushed the existing runtime check in > gup_must_unshare() to be more generic. > > IIUC it's also a matter of whether you'd want PageAnonExclusive() to take > care of both thp + hugetlb in one shot, rather than let callers handle it > by things like "if (PageHuge()) ... else ...", which I would try to avoid.
I tried to not let the caller pass in things that didn't make any sense.
Getting a tail page on a hugetlb folio in a page table walker except GUP-fast was completely bogus before your patch.
PageAnonExclusive was designed to be set on the page that was pointed to by a PTE, like having an additional PTE bit. Cont-pte/cont-pmd with the hugetlb fuzz around it we all love (huge_pte_offset()) did the right thing, because it abstracted the "multiple cont-pte/cont-pmd" PTEs to just a single logical PTE, with a single dedicated PageAnonExclusive.
So "conceptually", the caller that knows how the "single logical PTE" was the one to handle it. That meant, GUP-fast needed to be special, because it was unaware of the huge_pte_offset() logic.
But that seems to change now as we are changing our page table walkers, so I don't particularly care how we handle it.
> It seems so far cleaner to allow PageAnonExclusive() take whatever tail > pages, thp or hugetlb. But maybe your ultimate patchset can be even better > than that.
At least that part will be much cleaner.
-- Cheers,
David / dhildenb
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