On 2022/5/28 11:13, Matthew Wilcox wrote:
> On Sat, May 28, 2022 at 10:52:11AM +0800, Miaohe Lin wrote:
>> On 2022/5/27 23:02, Matthew Wilcox wrote:
>>> What? No. This can absolutely happen. We have a refcount on the folio,
>>> which means that any other thread can temporarily raise the refcount,
>>
>> IIUC, the folio is only in the isolated page_list now and it's not in the page cache, swap cache, pagetable or
>> under any use. So there should be no way that any other thread can temporarily raise the refcount when
>> folio_ref_count == 1. Or am I miss something?
>
> Take a look at something like GUP (fast). If this page _was_ mapped to
> userspace, something like this can happen:
>
> Thread A Thread B
> load PTE
> unmap page
> refcount goes to 1
> vmscan sees the page
> try_get_ref
> refcount is now 2. WARN_ON.
>
> Thread A will see that the PTE has changed and will now drop its
> reference, but Thread B already spat out the WARN.
>
> A similar thing can happen with the page cache.
Oh, I see. Many thanks for your patient explanation! :)
>
> If this is a worthwhile optimisation (does it happen often that we find
> a refcount == 1 page?), we could do something like ...
No, It should be rare.
>
> if (folio_ref_freeze(folio, 1)) {
> nr_pages = folio_nr_pages(folio);
> goto free_it;
> }
>
> ... or ...
>
> if (folio_ref_count(folio) == 1 &&
> folio_ref_freeze(folio, 1)) {
>
> ... if we want to test-and-test-and-clear
These proposed code changes look good to me.
>
> But this function is far too complicated already. I really want to
> see numbers that proves the extra complexity is worth it.
This optimization can save lots of cpu cycles and avoid possible disk I/O in
that specified case. But that is a somewhat rare case. So there's no numbers
that proves the extra complexity is worth it.
Should I drop this patch or proceed with the proposed code changes above in
next version? :)
Many thanks!
>
>
> .
>
