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

[Yang Shi]

On Wed, Aug 19, 2020 at 1:15 PM Gao Xiang <hsiangkao@xxxxxxxxxx> wrote:
>
> Hi Andrew,
>
> 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 :(
>
> Thanks for your reply...
>
> The related logic is in __swap_writepage() and setup_swap_extents(),
> and also see e.g generic_swapfile_activate() or iomap_swapfile_activate()...

I think just NFS falls into this case, so you may rephrase it to:

SWP_FS is only used for swap files over NFS. So, !SWP_FS means non NFS
swap, it could be either file backed or device backed.

Does this look more understandable?

> I will also talk with "Huang, Ying" in person if no response here.
>
> >
> > > 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?
>
> I'm not sure if the Redhat BZ is available for public, it can be reproduced
> since rhel 8
> https://bugzilla.redhat.com/show_bug.cgi?id=1855474
>
> It seems hard to believe, but I think just because rare user uses the SSD device +
> THP + file-backed swap device combination... maybe I'm wrong here, but my test
> shows as it is.
>
> Thanks,
> Gao Xiang
>
> >
> >
> >
>
>