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Message-ID: <9d72f2ed-9a92-e67b-3af5-79050004a1a4@nvidia.com>
Date:   Tue, 27 Sep 2022 19:57:29 -0700
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Subject: Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap()
 and _move()
Content-Language: en-US
To:     Yang Shi <shy828301@gmail.com>
Cc:     "Huang, Ying" <ying.huang@intel.com>,
        Alistair Popple <apopple@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Zi Yan <ziy@nvidia.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Oscar Salvador <osalvador@suse.de>,
        Matthew Wilcox <willy@infradead.org>
References: <20220921060616.73086-1-ying.huang@intel.com>
 <20220921060616.73086-3-ying.huang@intel.com>
 <87o7v2lbn4.fsf@nvdebian.thelocal>
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From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <CAHbLzkoEDUauo-H=zYvnDTC8TX4uezPxA4nV=QVQK_qxyZ3wjQ@mail.gmail.com>
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On 9/27/22 19:14, Yang Shi wrote:
> IIRC, the writeback may not call generic_writepages. On my ext4
> filesystem, the writeback call stack looks like:
> 
> @[
>     ext4_writepages+1
>     do_writepages+191
>     __writeback_single_inode+65
>     writeback_sb_inodes+477
>     __writeback_inodes_wb+76
>     wb_writeback+457
>     wb_workfn+680
>     process_one_work+485
>     worker_thread+80
>     kthread+231
>     ret_from_fork+34
> ]: 2
> 

Sure, that's fine for ext4, in that particular case, but

a) not all filesystems have .writepages hooked up. That's why
do_writepages() checks for .writepages(), and falls back to
.writepage():

	if (mapping->a_ops->writepages)
		ret = mapping->a_ops->writepages(mapping, wbc);
	else
		ret = generic_writepages(mapping, wbc);

, and

b) there are also a lot of places and ways to invoke writebacks. There
are periodic writebacks, and there are data integrity (WB_SYNC_ALL)
writebacks, and other places where a page needs to be cleaned--so, a lot
of call sites. Some of which will land on a .writepage() sometimes, even
now.

For just one example, I see migrate.c's writeout() function directly
calling writepage():

	rc = mapping->a_ops->writepage(&folio->page, &wbc);


thanks,

-- 
John Hubbard
NVIDIA