Re: mlockall(MCL_CURRENT) blocking infinitely

[Johannes Weiner]

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.

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?