Re: [PATCH] mm,thp: recheck each page before collapsing file THP

[Rik van Riel <riel@fb.com>]

On Fri, 2019-10-18 at 16:34 +0300, Kirill A. Shutemov wrote:
> On Thu, Oct 17, 2019 at 10:08:32PM -0700, Song Liu wrote:
> > In collapse_file(), after locking the page, it is necessary to
> > recheck
> > that the page is up-to-date, clean, and pointing to the proper
> > mapping.
> > If any check fails, abort the collapse.
> > 
> > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-
> > shmem) FS")
> > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Hugh Dickins <hughd@google.com>
> > Cc: William Kucharski <william.kucharski@oracle.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Signed-off-by: Song Liu <songliubraving@fb.com>
> > ---
> >  mm/khugepaged.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 0a1b4b484ac5..7da49b643c4d 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1619,6 +1619,14 @@ static void collapse_file(struct mm_struct
> > *mm,
> >  				result = SCAN_PAGE_LOCK;
> >  				goto xa_locked;
> >  			}
> > +
> > +			/* double check the page is correct and clean
> > */
> > +			if (unlikely(!PageUptodate(page)) ||
> > +			    unlikely(PageDirty(page)) ||
> > +			    unlikely(page->mapping != mapping)) {
> > +				result = SCAN_FAIL;
> > +				goto out_unlock;
> > +			}
> >  		}
> >  
> >  		/*
> 
> Hm. But why only for !is_shmem? Or I read it wrong?

It looks like the shmem code path has its own way of bailing
out when a page is !PageUptodate. Also, shmem can handle dirty
pages fine.

However, I suppose the shmem code might want to check for truncated
pages, which it does not curretnly appear to do. I guess doing
the trylock_page under the xarray lock may protect against truncate,
but that is subtle enough that at the very least it should be
documented.