linux-kernel - Re: [LTP] [mm/page] ab19939a6a: ltp.msync04.fail

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Date:   Mon, 13 Sep 2021 10:11:22 +0200
From:   Cyril Hrubis <chrubis@...e.cz>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     Chi Wu <wuchi.zero@...il.com>,
        Miklos Szeredi <mszeredi@...hat.com>, Jan Kara <jack@...e.cz>,
        lkp@...el.com, lkp@...ts.01.org,
        LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...com>,
        Sedat Dilek <sedat.dilek@...il.com>, Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        ltp@...ts.linux.it
Subject: Re: [LTP] [mm/page]  ab19939a6a: ltp.msync04.fail

Hi!
> FYI, we noticed the following commit (built with gcc-9):
> 
> commit: ab19939a6a5010cba4e9cb04dd8bee03c72edcbd ("mm/page-writeback: Fix performance when BDI's share of ratio is 0.")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> 
> in testcase: ltp
> version: ltp-x86_64-14c1f76-1_20210907
> with following parameters:
> 
> 	disk: 1HDD
> 	fs: xfs
> 	test: syscalls-03
> 	ucode: 0xe2
> 
> test-description: The LTP testsuite contains a collection of tools for testing the Linux kernel and related features.
> test-url: http://linux-test-project.github.io/

The msync04 test formats a device with a diffrent filesystems, for each
filesystem it maps a file, writes to the mapped page and the checks a
dirty bit in /proc/kpageflags before and after msync() on that page.

This seems to be broken after this patch for ntfs over FUSE and it looks
like the page does not have a dirty bit set right after it has been
written to.

Also I guess that we should increase the number of the pages we dirty or
attempt to retry since a single page may be flushed to the storage if we
are unlucky and the process is preempted between the write and the
initial check for the dirty bit.

-- 
Cyril Hrubis
chrubis@...e.cz