linux-kernel - Re: [PATCH] mm/vmscan: don't try to reclaim freed folios

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Date:   Sat, 28 May 2022 14:24:34 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/vmscan: don't try to reclaim freed folios

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!

> 
> 
> .
>