On Tue 05-11-19 13:22:11, Johannes Weiner wrote: > On Tue, Nov 05, 2019 at 04:28:21PM +0100, Vlastimil Babka wrote: > > On 11/5/19 2:23 PM, Robert Stupp wrote: > > > "git bisect" led to a result. > > > > > > The offending merge commit is f91f2ee54a21404fbc633550e99d69d14c2478f2 > > > "Merge branch 'akpm' (rest of patches from Andrew)". > > > > > > The first bad commit in the merged series of commits is > > > https://github.com/torvalds/linux/commit/6b4c9f4469819a0c1a38a0a4541337e0f9bf6c11 > > > . a75d4c33377277b6034dd1e2663bce444f952c14, the commit before 6b4c9f44, > > > is good. > > > > Ah, great you could bisect this. CCing people from the commit > > 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations") > > Judging from Robert's stack captures, the task is not hung but > busy-looping in __mm_populate(). AFAICS, the only way this can occur > is if populate_vma_page_range() returns 0 and we don't advance the > iteration position (if it returned an error, we wouldn't reset nend > and move on to the next vma as ignore_errors is 1 for mlockall.) > > populate_vma_page_range() returns 0 when the first page is not found > and faultin_page() returns -EBUSY (if it were processing pages, or if > the error from faultin_page() would be a different one, we would > return the number of pages processed or -error). > > faultin_page() returns -EBUSY when VM_FAULT_RETRY is set, i.e. we > dropped the mmap_sem in order to initiate IO and require a retry. That > is consistent with the bisect result (new VM_FAULT_RETRY conditions). > > At this point, regular page fault would retry with FAULT_FLAG_TRIED to > indicate that the mmap_sem cannot be dropped a second time. But this > mlock path doesn't set that flag and we can loop repeatedly. That is > something we probably need to fix with a FOLL_TRIED somewhere. It seems we could use __get_user_pages_locked() for that in populate_vma_page_range() if we were guaranteed that mm stays alive. This is guaranteed for current->mm cases but there seem to be some callers to populate_vma_page_range() where mm could indeed go away once we drop mmap_sem. These luckily pass NULL for the 'nonblocking' parameter though so all call sites seem to be fine but it would be fragile... > What I don't quite understand yet is why the fault path doesn't make > progress eventually. We must drop the mmap_sem without changing the > state in any way. How can we keep looping on the same page? That may be a slight suboptimality with Josef's patches. If the page is marked as PageReadahead, we always drop mmap_sem if we can and start readahead without checking whether that makes sense or not in do_async_mmap_readahead(). OTOH page_cache_async_readahead() then clears PageReadahead so the only way how I can see we could loop like this is when file->ra->ra_pages is 0. Not sure if that's what's happening through. We'd need to find which of the paths in filemap_fault() calls maybe_unlock_mmap_for_io() to tell more. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
