Re: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative()

[Hugh Dickins]

On Fri, 10 Dec 2021, Andrew Morton wrote:
> On Wed, 8 Dec 2021 23:19:18 -0800 (PST) Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> > It is not easily reproducible, but on 5.16-rc I have several times hit
> > the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative():
> > usually from filemap_get_read_batch() for an ext4 read, yesterday from
> > next_uptodate_page() from filemap_map_pages() for a shmem fault.
> > 
> > That BUG used to be placed where page_ref_add_unless() had succeeded,
> > but now it is placed before folio_ref_add_unless() is attempted: that
> > is not safe, since it is only the acquired reference which makes the
> > page safe from racing THP collapse or split.
> > 
> > We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu()
> > has succeeded; but I don't think it adds much value - just delete it.
> > 
> > Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()")
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> 
> I added cc:stable to this.

Thanks, but no, cc:stable not needed: the fixed commit went into 5.16-rc1,
and did not go to stable itself. There was an identical VM_BUG_ON_PAGE in
the old __page_cache_add_speculative(), but that one was correctly placed,
so there's no need for the old one to be removed.

Hugh