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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 8:15 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Mon, Jun 24, 2019 at 03:04:21PM +0000, Song Liu wrote:
>>
>>
>>> 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.
>
> Atrifically slowing down IO should do the trick.
>

Let me try that.

Thanks,
Song

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