Messages in this thread | | | Date | Tue, 2 Apr 2024 18:39:31 +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 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.
-- Cheers,
David / dhildenb
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