linux-kernel - Re: [PATCH] mm: delete unsafe BUG from page_cache_add

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Message-ID: <7e783f99-148f-c7aa-08a-4c4c45c5506a@google.com>
Date:   Fri, 10 Dec 2021 11:18:59 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        William Kucharski <william.kucharski@...cle.com>,
        Christoph Hellwig <hch@....de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: delete unsafe BUG from
 page_cache_add_speculative()

On Fri, 10 Dec 2021, Andrew Morton wrote:
> On Wed, 8 Dec 2021 23:19:18 -0800 (PST) Hugh Dickins <hughd@...gle.com> 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@...gle.com>
> 
> 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