linux-kernel - Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap() and

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Date:   Tue, 27 Sep 2022 19:57:29 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Oscar Salvador <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap()
 and _move()

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