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

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Date:   Sat, 28 May 2022 04:13:44 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
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 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.

If this is a worthwhile optimisation (does it happen often that we find
a refcount == 1 page?), we could do something like ...

		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

But this function is far too complicated already.  I really want to
see numbers that proves the extra complexity is worth it.