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@redhat.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 :(
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