| Message ID | 20210930215311.240774-1-shy828301@gmail.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Solve silent data loss caused by poisoned page cache (shmem/tmpfs) | expand |
On Thu, Sep 30, 2021 at 02:53:06PM -0700, Yang Shi wrote: > Yang Shi (5): > mm: hwpoison: remove the unnecessary THP check > mm: filemap: check if THP has hwpoisoned subpage for PMD page fault > mm: hwpoison: refactor refcount check handling > mm: shmem: don't truncate page if memory failure happens > mm: hwpoison: handle non-anonymous THP correctly Today I just noticed one more thing: unpoison path has (unpoison_memory): if (page_mapping(page)) { unpoison_pr_info("Unpoison: the hwpoison page has non-NULL mapping %#lx\n", pfn, &unpoison_rs); return 0; } I _think_ it was used to make sure we ignore page that was not successfully poisoned/offlined before (for anonymous), so raising this question up on whether we should make sure e.g. shmem hwpoisoned pages still can be unpoisoned for debugging purposes.
On Tue, Oct 12, 2021 at 7:41 PM Peter Xu <peterx@redhat.com> wrote: > > On Thu, Sep 30, 2021 at 02:53:06PM -0700, Yang Shi wrote: > > Yang Shi (5): > > mm: hwpoison: remove the unnecessary THP check > > mm: filemap: check if THP has hwpoisoned subpage for PMD page fault > > mm: hwpoison: refactor refcount check handling > > mm: shmem: don't truncate page if memory failure happens > > mm: hwpoison: handle non-anonymous THP correctly > > Today I just noticed one more thing: unpoison path has (unpoison_memory): > > if (page_mapping(page)) { > unpoison_pr_info("Unpoison: the hwpoison page has non-NULL mapping %#lx\n", > pfn, &unpoison_rs); > return 0; > } > > I _think_ it was used to make sure we ignore page that was not successfully > poisoned/offlined before (for anonymous), so raising this question up on > whether we should make sure e.g. shmem hwpoisoned pages still can be unpoisoned > for debugging purposes. Yes, not only mapping, the refcount check is not right if page cache page is kept in page cache instead of being truncated after this series. But actually unpoison has been broken since commit 0ed950d1f28142ccd9a9453c60df87853530d778 ("mm,hwpoison: make get_hwpoison_page() call get_any_page()"). And Naoya said in the commit "unpoison_memory() is also unchanged because it's broken and need thorough fixes (will be done later)." I do have some fixes in my tree to unblock tests and fix unpoison for this series (just make it work for testing). Naoya may have some ideas in mind and it is just a debugging feature so I don't think it must be fixed in this series. It could be done later. I could add a TODO section in the cover letter to make this more clear. > > -- > Peter Xu >
On Tue, Oct 12, 2021 at 08:09:24PM -0700, Yang Shi wrote: > On Tue, Oct 12, 2021 at 7:41 PM Peter Xu <peterx@redhat.com> wrote: > > > > On Thu, Sep 30, 2021 at 02:53:06PM -0700, Yang Shi wrote: > > > Yang Shi (5): > > > mm: hwpoison: remove the unnecessary THP check > > > mm: filemap: check if THP has hwpoisoned subpage for PMD page fault > > > mm: hwpoison: refactor refcount check handling > > > mm: shmem: don't truncate page if memory failure happens > > > mm: hwpoison: handle non-anonymous THP correctly > > > > Today I just noticed one more thing: unpoison path has (unpoison_memory): > > > > if (page_mapping(page)) { > > unpoison_pr_info("Unpoison: the hwpoison page has non-NULL mapping %#lx\n", > > pfn, &unpoison_rs); > > return 0; > > } > > > > I _think_ it was used to make sure we ignore page that was not successfully > > poisoned/offlined before (for anonymous), so raising this question up on > > whether we should make sure e.g. shmem hwpoisoned pages still can be unpoisoned > > for debugging purposes. > > Yes, not only mapping, the refcount check is not right if page cache > page is kept in page cache instead of being truncated after this > series. But actually unpoison has been broken since commit > 0ed950d1f28142ccd9a9453c60df87853530d778 ("mm,hwpoison: make > get_hwpoison_page() call get_any_page()"). And Naoya said in the > commit "unpoison_memory() is also unchanged because it's broken and > need thorough fixes (will be done later)." > > I do have some fixes in my tree to unblock tests and fix unpoison for > this series (just make it work for testing). Naoya may have some ideas > in mind and it is just a debugging feature so I don't think it must be > fixed in this series. It could be done later. I could add a TODO > section in the cover letter to make this more clear. I see, that sounds good enough to me. Thanks,
On Tue, Oct 12, 2021 at 08:09:24PM -0700, Yang Shi wrote: > On Tue, Oct 12, 2021 at 7:41 PM Peter Xu <peterx@redhat.com> wrote: > > > > On Thu, Sep 30, 2021 at 02:53:06PM -0700, Yang Shi wrote: > > > Yang Shi (5): > > > mm: hwpoison: remove the unnecessary THP check > > > mm: filemap: check if THP has hwpoisoned subpage for PMD page fault > > > mm: hwpoison: refactor refcount check handling > > > mm: shmem: don't truncate page if memory failure happens > > > mm: hwpoison: handle non-anonymous THP correctly > > > > Today I just noticed one more thing: unpoison path has (unpoison_memory): > > > > if (page_mapping(page)) { > > unpoison_pr_info("Unpoison: the hwpoison page has non-NULL mapping %#lx\n", > > pfn, &unpoison_rs); > > return 0; > > } > > > > I _think_ it was used to make sure we ignore page that was not successfully > > poisoned/offlined before (for anonymous), so raising this question up on > > whether we should make sure e.g. shmem hwpoisoned pages still can be unpoisoned > > for debugging purposes. > > Yes, not only mapping, the refcount check is not right if page cache > page is kept in page cache instead of being truncated after this > series. But actually unpoison has been broken since commit > 0ed950d1f28142ccd9a9453c60df87853530d778 ("mm,hwpoison: make > get_hwpoison_page() call get_any_page()"). And Naoya said in the > commit "unpoison_memory() is also unchanged because it's broken and > need thorough fixes (will be done later)." > Yeah, I have a patch but still not make it merged to upstream. Sorry about that... > I do have some fixes in my tree to unblock tests and fix unpoison for > this series (just make it work for testing). Naoya may have some ideas > in mind and it is just a debugging feature so I don't think it must be > fixed in this series. It could be done later. I could add a TODO > section in the cover letter to make this more clear. Yes, it's fine to me that you leave this unsolved in this patchset. Thanks, Naoya Horiguchi
When discussing the patch that splits page cache THP in order to offline the poisoned page, Noaya mentioned there is a bigger problem [1] that prevents this from working since the page cache page will be truncated if uncorrectable errors happen. By looking this deeper it turns out this approach (truncating poisoned page) may incur silent data loss for all non-readonly filesystems if the page is dirty. It may be worse for in-memory filesystem, e.g. shmem/tmpfs since the data blocks are actually gone. To solve this problem we could keep the poisoned dirty page in page cache then notify the users on any later access, e.g. page fault, read/write, etc. The clean page could be truncated as is since they can be reread from disk later on. The consequence is the filesystems may find poisoned page and manipulate it as healthy page since all the filesystems actually don't check if the page is poisoned or not in all the relevant paths except page fault. In general, we need make the filesystems be aware of poisoned page before we could keep the poisoned page in page cache in order to solve the data loss problem. To make filesystems be aware of poisoned page we should consider: - The page should be not written back: clearing dirty flag could prevent from writeback. - The page should not be dropped (it shows as a clean page) by drop caches or other callers: the refcount pin from hwpoison could prevent from invalidating (called by cache drop, inode cache shrinking, etc), but it doesn't avoid invalidation in DIO path. - The page should be able to get truncated/hole punched/unlinked: it works as it is. - Notify users when the page is accessed, e.g. read/write, page fault and other paths (compression, encryption, etc). The scope of the last one is huge since almost all filesystems need do it once a page is returned from page cache lookup. There are a couple of options to do it: 1. Check hwpoison flag for every path, the most straightforward way. 2. Return NULL for poisoned page from page cache lookup, the most callsites check if NULL is returned, this should have least work I think. But the error handling in filesystems just return -ENOMEM, the error code will incur confusion to the users obviously. 3. To improve #2, we could return error pointer, e.g. ERR_PTR(-EIO), but this will involve significant amount of code change as well since all the paths need check if the pointer is ERR or not just like option #1. I did prototype for both #1 and #3, but it seems #3 may require more changes than #1. For #3 ERR_PTR will be returned so all the callers need to check the return value otherwise invalid pointer may be dereferenced, but not all callers really care about the content of the page, for example, partial truncate which just sets the truncated range in one page to 0. So for such paths it needs additional modification if ERR_PTR is returned. And if the callers have their own way to handle the problematic pages we need to add a new FGP flag to tell FGP functions to return the pointer to the page. It may happen very rarely, but once it happens the consequence (data corruption) could be very bad and it is very hard to debug. It seems this problem had been slightly discussed before, but seems no action was taken at that time. [2] As the aforementioned investigation, it needs huge amount of work to solve the potential data loss for all filesystems. But it is much easier for in-memory filesystems and such filesystems actually suffer more than others since even the data blocks are gone due to truncating. So this patchset starts from shmem/tmpfs by taking option #1. Patch #1: cleanup, depended by patch #2 Patch #2: fix THP with hwpoisoned subpage(s) PMD map bug Patch #2: refactor and preparation. Patch #4: keep the poisoned page in page cache and handle such case for all the paths. Patch #5: the previous patches unblock page cache THP split, so this patch add page cache THP split support. I didn't receive too many comments for patch #3 ~ #5, so may consider separate the bug fixes (patch #1 and #2) from others to make them merged sooner. This version still includes all 5 patches. Changelog v2 --> v3: * Incorporated the comments from Kirill. * Reordered the series to reflect the right dependency (patch #3 from v2 is patch #1 in this revision, patch #1 from v2 is patch #2 in this revision). * After the reorder, patch #2 depends on patch #1 and both need to be backported to -stable. v1 --> v2: * Incorporated the suggestion from Kirill to use a new page flag to indicate there is hwpoisoned subpage(s) in a THP. (patch #1) * Dropped patch #2 of v1. * Refctored the page refcount check logic of hwpoison per Naoya. (patch #2) * Removed unnecessary THP check per Naoya. (patch #3) * Incorporated the other comments for shmem from Naoya. (patch #4) Yang Shi (5): mm: hwpoison: remove the unnecessary THP check mm: filemap: check if THP has hwpoisoned subpage for PMD page fault mm: hwpoison: refactor refcount check handling mm: shmem: don't truncate page if memory failure happens mm: hwpoison: handle non-anonymous THP correctly include/linux/page-flags.h | 19 +++++++++++ mm/filemap.c | 12 +++---- mm/huge_memory.c | 2 ++ mm/memory-failure.c | 129 +++++++++++++++++++++++++++++++++++++++++++-------------------------- mm/memory.c | 9 +++++ mm/page_alloc.c | 4 ++- mm/shmem.c | 31 +++++++++++++++-- mm/userfaultfd.c | 5 +++ 8 files changed, 153 insertions(+), 58 deletions(-) [1] https://lore.kernel.org/linux-mm/CAHbLzkqNPBh_sK09qfr4yu4WTFOzRy+MKj+PA7iG-adzi9zGsg@mail.gmail.com/T/#m0e959283380156f1d064456af01ae51fdff91265 [2] https://lore.kernel.org/lkml/20210318183350.GT3420@casper.infradead.org/