Re: [PATCH v2] mm, THP, swap: fix allocating cluster for swapfile by mistake

[Gao Xiang]

Hi Matthew,

On Thu, Aug 20, 2020 at 12:34:48PM +0100, Matthew Wilcox wrote:
> On Thu, Aug 20, 2020 at 12:53:23PM +0800, Gao Xiang wrote:
> > SWP_FS is used to make swap_{read,write}page() go through
> > the filesystem, and it's only used for swap files over
> > NFS. So, !SWP_FS means non NFS for now, it could be either
> > file backed or device backed. Something similar goes with
> > legacy SWP_FILE.
> > 
> > So in order to achieve the goal of the original patch,
> > SWP_BLKDEV should be used instead.
> 
> This is clearly confusing.  I think we need to rename SWP_FS to SWP_FS_OPS.
> 
> More generally, the swap code seems insane.  I appreciate that it's an
> inherited design from over twenty-five years ago, and nobody wants to
> touch it, but it's crazy that it cares about how the filesystem has
> mapped file blocks to disk blocks.  I understand that the filesystem
> has to know not to allocate memory in order to free memory, but this
> is already something filesystems have to understand.  It's also useful
> for filesystems to know that this is data which has no meaning after a
> power cycle (so it doesn't need to be journalled or snapshotted or ...),
> but again, that's useful functionality which we could stand to present
> to userspace anyway.
> 
> I suppose the tricky thing about it is that working on the swap code is
> not as sexy as working on a filesystem, and doing the swap code right
> is essentially writing a filesystem, so everybody who's capable already
> has something better to do.

Yeah, I agree with your point. After looking into swap code a bit
(swapfile.c and swap.c), I think such code really needs to be
cleaned up... But I'm lack of motivation about this since I couldn't
guarantee to introduce some new regression and honestly I don't care
much about this piece of code.

Maybe some new projects based on this could help clean up that
as well. :)

Anyway, we really need a quick fix to avoid such FS corruption,
which looks dangerous on the consumer side.

> 
> Anyway, Gao, please can you submit a follow-on patch to rename SWP_FS?

Ok, anyway, that is another stuff and may need some other thread.
I will seek some time to send out a patch for further discussion later.

Thanks,
Gao Xiang

>