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

[Rafael Aquini]

On Wed, Aug 19, 2020 at 01:05:06PM -0700, Andrew Morton wrote:
> On Thu, 20 Aug 2020 03:56:13 +0800 Gao Xiang <hsiangkao@xxxxxxxxxx> 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 :(
> 

I'll work with Gao to rephrase that message. Sorry!


> > 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?
>

My bet here is that it's rare to go for a swapfile on non-rotational
devices, and even rarer to create the swapfile when the filesystem is
already fragmented. 
 
RHEL-8, v4.18-based, is starting to see more adpters among Red Hat's
customer base, thus the report now. We are also working on a secondary 
issue related to CONFIG_THP_SWAP, as well, where the deferred THP split
registered shriker goes for a NULL pointer dereference in case the
swap device is backed by a rotational drive.

-- Rafael