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SubjectRe: [PATCH 05/31] mm/filemap: allow pte_offset_map_lock() to fail
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On 2023/5/24 10:35, Hugh Dickins wrote:
> 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.

Got it. Thanks for the explanation.

>
> Hugh

--
Thanks,
Qi

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