linux-kernel - Re: summarize all information again at bottom//reply: reply: [PATCH] mm: fix a race scenario in folio_isolate

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Date: Wed, 27 Mar 2024 09:25:59 +0800
From: Zhaoyang Huang <huangzhaoyang@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: 黄朝阳 (Zhaoyang Huang) <zhaoyang.huang@...soc.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	康纪滨 (Steve Kang) <Steve.Kang@...soc.com>
Subject: Re: summarize all information again at bottom//reply: reply: [PATCH]
 mm: fix a race scenario in folio_isolate_lru

On Tue, Mar 26, 2024 at 8:21 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Tue, Mar 26, 2024 at 05:06:55PM +0800, Zhaoyang Huang wrote:
> > 1. Thread_readahead remove the folio from page cache and drop 2 refcnt
> > by readahead_folio & filemap_remove_folio(get rid of the folios which
> > failed to launch IO during readahead)
> >     refcnt == 0, PG_lru == true, PG_lock == true
> >     read_pages
> > ...
> >         folio = readahead_folio
> >         <one refcnt dropped here>
> > ********For the folio which can not launch IO, we should NOT drop
> > refcnt here??? replaced by __readahead_folio???**********
> >         folio_get
> >         filemap_remove_folio(folio)
> >         folio_unlock
> >         <one refcnt dropped here>
> >         folio_put
>
> Ignoring any other thread, you're basically saying that there's a
> refcount imbalance here.  Which means we'd hit an assert (that folio
> refcount went below zero) in the normal case where another thread wasn't
> simultaneously trying to do anything.
Theoretically Yes but it is rare in practice as aops->readahead will
launch all pages to IO under most scenarios.

read_pages
    aops->readahead[1]
..
    while (folio = readahead_folio)[2]
        filemap_remove_folio

IMO, according to the comments of readahead_page, the refcnt
represents page cache dropped in [1] makes sense for two reasons, '1.
The folio is going to do IO and is locked until IO done;2. The refcnt
will be added back when found again from the page cache and then serve
for PTE or vfs' while it doesn't make sense in [2] as the refcnt of
page cache will be dropped in filemap_remove_folio

 * Context: The page is locked and has an elevated refcount.  The caller
 * should decreases the refcount once the page has been submitted for I/O
 * and unlock the page once all I/O to that page has completed.
 * Return: A pointer to the next page, or %NULL if we are done.