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Date:   Tue, 23 May 2023 19:35:42 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Qi Zheng <qi.zheng@linux.dev>
cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
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        Suren Baghdasaryan <surenb@google.com>,
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        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 05/31] mm/filemap: allow pte_offset_map_lock() to fail
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On Mon, 22 May 2023, Qi Zheng wrote:
> On 2023/5/22 12:54, Hugh Dickins wrote:
> > filemap_map_pages() allow pte_offset_map_lock() to fail; and remove the
> > pmd_devmap_trans_unstable() check from filemap_map_pmd(), which can safely
> > return to filemap_map_pages() and let pte_offset_map_lock() discover that.
> > 
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > ---
> >   mm/filemap.c | 12 +++++-------
> >   1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 28b42ee848a4..9e129ad43e0d 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -3408,13 +3408,6 @@ static bool filemap_map_pmd(struct vm_fault *vmf,
> > struct folio *folio,
> >    if (pmd_none(*vmf->pmd))
> >     pmd_install(mm, vmf->pmd, &vmf->prealloc_pte);
> >   -	/* See comment in handle_pte_fault() */
> > -	if (pmd_devmap_trans_unstable(vmf->pmd)) {
> > -		folio_unlock(folio);
> > -		folio_put(folio);
> > -		return true;
> > -	}
> > -
> >   	return false;
> >   }
> >   
> > @@ -3501,6 +3494,11 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
> >   
> >    addr = vma->vm_start + ((start_pgoff - vma->vm_pgoff) << PAGE_SHIFT);
> >    vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
> > +	if (!vmf->pte) {
> > +		folio_unlock(folio);
> > +		folio_put(folio);
> 
> Missing:
> 		ret = VM_FAULT_NOPAGE;
> ?

No, not missed.  Here ret is 0, which leads do_read_fault() to try
__do_fault() afterwards.  Whereas VM_FAULT_NOPAGE would send it back
to userspace to retry the whole fault.  Either will work, but I think
the intention of VM_FAULT_NOPAGE here in filemap_map_pages() is to say
"the page you want is now inserted", which is probably not the case.

Hugh