Messages in this thread | | | Date | Thu, 23 Mar 2023 11:38:52 +0100 | Subject | Re: [PATCH v7 4/4] mm: vmalloc: convert vread() to vread_iter() | From | David Hildenbrand <> |
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On 23.03.23 11:36, Baoquan He wrote: > On 03/23/23 at 06:44am, Lorenzo Stoakes wrote: >> On Thu, Mar 23, 2023 at 10:52:09AM +0800, Baoquan He wrote: >>> On 03/22/23 at 06:57pm, Lorenzo Stoakes wrote: >>>> Having previously laid the foundation for converting vread() to an iterator >>>> function, pull the trigger and do so. >>>> >>>> This patch attempts to provide minimal refactoring and to reflect the >>>> existing logic as best we can, for example we continue to zero portions of >>>> memory not read, as before. >>>> >>>> Overall, there should be no functional difference other than a performance >>>> improvement in /proc/kcore access to vmalloc regions. >>>> >>>> Now we have eliminated the need for a bounce buffer in read_kcore_iter(), >>>> we dispense with it, and try to write to user memory optimistically but >>>> with faults disabled via copy_page_to_iter_nofault(). We already have >>>> preemption disabled by holding a spin lock. We continue faulting in until >>>> the operation is complete. >>> >>> I don't understand the sentences here. In vread_iter(), the actual >>> content reading is done in aligned_vread_iter(), otherwise we zero >>> filling the region. In aligned_vread_iter(), we will use >>> vmalloc_to_page() to get the mapped page and read out, otherwise zero >>> fill. While in this patch, fault_in_iov_iter_writeable() fault in memory >>> of iter one time and will bail out if failed. I am wondering why we >>> continue faulting in until the operation is complete, and how that is done. >> >> This is refererrring to what's happening in kcore.c, not vread_iter(), >> i.e. the looped read/faultin. >> >> The reason we bail out if failt_in_iov_iter_writeable() is that would >> indicate an error had occurred. >> >> The whole point is to _optimistically_ try to perform the operation >> assuming the pages are faulted in. Ultimately we fault in via >> copy_to_user_nofault() which will either copy data or fail if the pages are >> not faulted in (will discuss this below a bit more in response to your >> other point). >> >> If this fails, then we fault in, and try again. We loop because there could >> be some extremely unfortunate timing with a race on e.g. swapping out or >> migrating pages between faulting in and trying to write out again. >> >> This is extremely unlikely, but to avoid any chance of breaking userland we >> repeat the operation until it completes. In nearly all real-world >> situations it'll either work immediately or loop once. > > Thanks a lot for these helpful details with patience. I got it now. I was > mainly confused by the while(true) loop in KCORE_VMALLOC case of read_kcore_iter. > > Now is there any chance that the faulted in memory is swapped out or > migrated again before vread_iter()? fault_in_iov_iter_writeable() will > pin the memory? I didn't find it from code and document. Seems it only > falults in memory. If yes, there's window between faluting in and > copy_to_user_nofault(). >
See the documentation of fault_in_safe_writeable():
"Note that we don't pin or otherwise hold the pages referenced that we fault in. There's no guarantee that they'll stay in memory for any duration of time."
-- Thanks,
David / dhildenb
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