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From: Zhaoyang Huang <huangzhaoyang@gmail.com>
Date: Thu, 28 Mar 2024 12:03:02 +0800
Message-ID: <CAGWkznEMCXQSe10E-pbdxk2uFgQO038wH6g=iojtSU6-N+GJdg@mail.gmail.com>
Subject: Re: summarize all information again at bottom//reply: reply: [PATCH]
 mm: fix a race scenario in folio_isolate_lru
To: Matthew Wilcox <willy@infradead.org>
Cc: =?UTF-8?B?6buE5pyd6ZizIChaaGFveWFuZyBIdWFuZyk=?= <zhaoyang.huang@unisoc.com>, 
	Andrew Morton <akpm@linux-foundation.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, 
	=?UTF-8?B?5bq357qq5ruoIChTdGV2ZSBLYW5nKQ==?= <Steve.Kang@unisoc.com>
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On Thu, Mar 28, 2024 at 11:18=E2=80=AFAM Matthew Wilcox <willy@infradead.or=
g> wrote:
>
> On Thu, Mar 28, 2024 at 09:27:31AM +0800, Zhaoyang Huang wrote:
> > ok, I missed the refcnt from alloc_pages. However, I still think it is
> > a bug to call readahead_folio in read_pages as the refcnt obtained by
> > alloc_pages should be its final guard which is paired to the one which
> > checked in shrink_folio_list->__remove_mapping->folio_ref_freeze(2)(thi=
s
> > 2 represent alloc_pages & page cache). If we removed this one without
>
> __remove_mapping()  requires that the caller holds the folio locked.
> Since the readahead code unlocks the folio, __remove_mapping() cannot
> be run because the caller of __remove_mapping() will wait for the folio
> lock.
repost the whole timing sequence to make it more clear and fix
incorrect description of previous feedback

Follow the refcount through.

In page_cache_ra_unbounded():

                folio =3D filemap_alloc_folio(gfp_mask, 0);
(folio has refcount 1)
                ret =3D filemap_add_folio(mapping, folio, index + i, gfp_ma=
sk);
(folio has refcount 2, PG_lru)

Then we call read_pages()
First we call ->readahead() which for some reason stops early.
Then we call readahead_folio() which calls folio_put()
(folio has refcount 1)
Then we call folio_get()
(folio has refcount 2)
Then we call filemap_remove_folio()
(folio has refcount 1)
Then we call folio_unlock()
Then we call folio_put()

Amending steps for previous timing sequence below where [1] races with
[2] that has nothing to do with __remove_mapping(). IMO, no file_folio
should be freed by folio_put as the refcnt obtained by alloc_pages
keep it always imbalanced until shrink_folio_list->__remove_mapping,
where the folio_ref_freeze(2) implies the refcnt of alloc_pages and
isolation should be the last two. release_pages is a special scenario
that the refcnt of alloc_pages is freed implicitly in
delete_from_page_cache_batch->filemap_free_folio.

    folio_put()
    {
         if(folio_put_test_zero())
*** we should NOT be here as the refcnt of alloc_pages should NOT be droppe=
d ***
         if (folio_test_lru())
***  preempted here with refcnt =3D=3D 0 and pass PG_lru check ***
[1]
         lruvec_del_folio()
Then thread_isolate call folio_isolate_lru()
      folio_isolate_lru()
      {
         folio_test_clear_lru()
         folio_get()
[2]
         lruvec_del_folio()
      }
---------------------------------------------------------------------------=
-----------------
shrink_folio_list()
{
    __remove_mapping()
    {
        refcount =3D 1 + folio_nr_pages;
*** the refcount =3D 1 + 1 implies there should be only the refcnt of
alloc_pages and previous isolation for a no-busy folio as all PTE has
gone***
        if (!folio_ref_freeze(refcount))
             goto keeplock;
     }
}