linux-kernel - Re: [PATCH] mm, THP, swap: fix allocating cluster for swapfile by mistake

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Message-Id: <20200819130506.eea076dd618644cd7ff875b6@linux-foundation.org>
Date:   Wed, 19 Aug 2020 13:05:06 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Gao Xiang <hsiangkao@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Rafael Aquini <aquini@...hat.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        Eric Sandeen <esandeen@...hat.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] mm, THP, swap: fix allocating cluster for swapfile by
 mistake

On Thu, 20 Aug 2020 03:56:13 +0800 Gao Xiang <hsiangkao@...hat.com> wrote:

> SWP_FS doesn't mean the device is file-backed swap device,
> which just means each writeback request should go through fs
> by DIO. Or it'll just use extents added by .swap_activate(),
> but it also works as file-backed swap device.

This is very hard to understand :(

> So in order to achieve the goal of the original patch,
> SWP_BLKDEV should be used instead.
> 
> FS corruption can be observed with SSD device + XFS +
> fragmented swapfile due to CONFIG_THP_SWAP=y.
> 
> Fixes: f0eea189e8e9 ("mm, THP, swap: Don't allocate huge cluster for file backed swap device")
> Fixes: 38d8b4e6bdc8 ("mm, THP, swap: delay splitting THP during swap out")

Why do you think it has taken three years to discover this?