On Thu, Aug 26, 2021 at 8:57 PM HORIGUCHI NAOYA(堀口 直也) <naoya.horiguchi@xxxxxxx> wrote: > > On Thu, Aug 26, 2021 at 03:03:57PM -0700, Yang Shi wrote: > > On Thu, Aug 26, 2021 at 1:03 PM Yang Shi <shy828301@xxxxxxxxx> wrote: > > > > > > On Wed, Aug 25, 2021 at 11:17 PM HORIGUCHI NAOYA(堀口 直也) > > > <naoya.horiguchi@xxxxxxx> wrote: > > > > > > > > On Tue, Aug 24, 2021 at 03:13:22PM -0700, Yang Shi wrote: > ... > > > > > > > > There was a discussion about another approach of keeping error pages in page > > > > cache for filesystem without backend storage. > > > > https://lore.kernel.org/lkml/alpine.LSU.2.11.2103111312310.7859@eggly.anvils/ > > > > This approach seems to me less complicated, but one concern is that this > > > > change affects user-visible behavior of memory errors. Keeping error pages > > > > in page cache means that the errors are persistent until next system reboot, > > > > so we might need to define the way to clear the errors to continue to use > > > > the error file. Current implementation is just to send SIGBUS to the > > > > mapping processes (at least once), then forget about the error, so there is > > > > no such issue. > > > > > > > > Another thought of possible solution might be to send SIGBUS immediately when > > > > a memory error happens on a shmem thp. We can find all the mapping processes > > > > before splitting shmem thp, so send SIGBUS first, then split it and contain > > > > the error page. This is not elegant (giving up any optional actions) but > > > > anyway we can avoid the silent data lost. > > > > > > Thanks a lot. I apologize I didn't notice you already posted a similar > > > patch before. > > > > > > Yes, I think I focused on the soft offline part too much and missed > > > the uncorrected error part and I admit I did underestimate the > > > problem. > > > > > > I think Hugh's suggestion makes sense if we treat tmpfs as a regular > > > filesystem (just memory backed). AFAIK, some filesystem, e.g. btrfs, > > > may do checksum after reading from storage block then return an error > > > if checksum is not right since it may indicate hardware failure on > > > disk. Then the syscalls or page fault return error or SIGBUS. > > > > > > So in shmem/tmpfs case, if hwpoisoned page is met, just return error > > > (-EIO or whatever) for syscall or SIGBUS for page fault. It does align > > > with the behavior of other filesystems. It is definitely applications' > > > responsibility to check the return value of read/write syscalls. > > > > BTW, IIUC the dirty regular page cache (storage backed) would be left > > in the page cache too, the clean page cache would be truncated since > > they can be just reread from storage, right? > > A dirty page cache is also removed on error (me_pagecache_dirty() falls > through me_pagecache_clean(), then truncate_error_page() is called). > The main purpose of this is to separate off the error page from exising > data structures to minimize the risk of later accesses (maybe by race or bug). > But we can change this behavior for specific file systems by updating > error_remove_page() callbacks in address_space_operation. Yeah, if fs's error_remove_page() is defined. It seems the filesystems which have error_remove_page() defined just use generic_remove_page() except hugetlbfs. And the generic implementation just clears the dirty flag and removes the page from page cache. If error_remove_page() is not defined, the page would stay in page cache since invalidate_inode_page() can't remove dirty page. > > Honestly, it seems to me that how dirty data is lost does not depend on > file system, and I'm still not sure that this is really a right approach > for the current issue. IMHO the biggest problem is that applications may see obsolete/inconsistent data silently, right? Actually keeping the corrupted page in page cache should be able to notify applications that they are accessing inconsistent data. > > Thanks, > Naoya Horiguchi
