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SubjectRe: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem) FS
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> On Jun 24, 2019, at 7:54 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Mon, Jun 24, 2019 at 02:42:13PM +0000, Song Liu wrote:
>>
>>
>>> On Jun 24, 2019, at 7:27 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>>>
>>> On Mon, Jun 24, 2019 at 02:01:05PM +0000, Song Liu wrote:
>>>>>> @@ -1392,6 +1403,23 @@ static void collapse_file(struct mm_struct *mm,
>>>>>> result = SCAN_FAIL;
>>>>>> goto xa_unlocked;
>>>>>> }
>>>>>> + } else if (!page || xa_is_value(page)) {
>>>>>> + xas_unlock_irq(&xas);
>>>>>> + page_cache_sync_readahead(mapping, &file->f_ra, file,
>>>>>> + index, PAGE_SIZE);
>>>>>> + lru_add_drain();
>>>>>
>>>>> Why?
>>>>
>>>> isolate_lru_page() is likely to fail if we don't drain the pagevecs.
>>>
>>> Please add a comment.
>>
>> Will do.
>>
>>>
>>>>>> + page = find_lock_page(mapping, index);
>>>>>> + if (unlikely(page == NULL)) {
>>>>>> + result = SCAN_FAIL;
>>>>>> + goto xa_unlocked;
>>>>>> + }
>>>>>> + } else if (!PageUptodate(page)) {
>>>>>
>>>>> Maybe we should try wait_on_page_locked() here before give up?
>>>>
>>>> Are you referring to the "if (!PageUptodate(page))" case?
>>>
>>> Yes.
>>
>> I think this case happens when another thread is reading the page in.
>> I could not think of a way to trigger this condition for testing.
>>
>> On the other hand, with current logic, we will retry the page on the
>> next scan, so I guess this is OK.
>
> What I meant that calling wait_on_page_locked() on !PageUptodate() page
> will likely make it up-to-date and we don't need to SCAN_FAIL the attempt.
>

Yeah, I got the point. My only concern is that I don't know how to
reliably trigger this case for testing. I can try to trigger it. But I
don't know whether it will happen easily.

Thanks,
Song




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