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

[Dave Chinner]

On Fri, Aug 21, 2020 at 08:21:45AM +0800, Gao Xiang wrote:
> Hi Dave,
> 
> On Fri, Aug 21, 2020 at 09:34:46AM +1000, Dave Chinner 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.
> > > 
> > > FS corruption can be observed with SSD device + XFS +
> > > fragmented swapfile due to CONFIG_THP_SWAP=y.
> > > 
> > > I reproduced the issue with the following details:
> > > 
> > > Environment:
> > > QEMU + upstream kernel + buildroot + NVMe (2 GB)
> > > 
> > > Kernel config:
> > > CONFIG_BLK_DEV_NVME=y
> > > CONFIG_THP_SWAP=y
> > 
> > Ok, so at it's core this is a swap file extent versus THP swap
> > cluster alignment issue?
> 
> I think yes.
> 
> > 
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index 6c26916e95fd..2937daf3ca02 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -1074,7 +1074,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> > >  			goto nextsi;
> > >  		}
> > >  		if (size == SWAPFILE_CLUSTER) {
> > > -			if (!(si->flags & SWP_FS))
> > > +			if (si->flags & SWP_BLKDEV)
> > >  				n_ret = swap_alloc_cluster(si, swp_entries);
> > >  		} else
> > >  			n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> > 
> > IOWs, if you don't make this change, does the corruption problem go
> > away if you align swap extents in iomap_swapfile_add_extent() to
> > (SWAPFILE_CLUSTER * PAGE_SIZE) instead of just PAGE_SIZE?
> > 
> > I.e. if the swapfile extents are aligned correctly to huge page swap
> > cluster size and alignment, does the swap clustering optimisations
> > for swapping THP pages work correctly? And, if so, is there any
> > performance benefit we get from enabling proper THP swap clustering
> > on swapfiles?
> > 
> 
> Yeah, I once think about some similiar thing as well. My thought for now is
> 
>  - First, SWAP THP doesn't claim to support such swapfile for now.
>    And the original author tried to explicitly avoid the whole thing in
> 
>    f0eea189e8e9 ("mm, THP, swap: Don't allocate huge cluster for file backed swap device")
> 
>    So such thing would be considered as some new feature and need
>    more testing at least. But for now I think we just need a quick
>    fix to fix the commit f0eea189e8e9 to avoid regression and for
>    backport use.

Sure, a quick fix is fine for the current issue. I'm asking
questions about the design/architecture of how THP_SWAP is supposed
to work and whether swapfiles are violating some other undocumented
assumption about swapping THP files...

>  - It is hard for users to control swapfile in
>    SWAPFILE_CLUSTER * PAGE_SIZE extents, especially users'
>    disk are fragmented or have some on-disk metadata limitation or
>    something. It's very hard for users to utilize this and arrange
>    their swapfile physical addr alignment and fragments for now.

This isn't something users control. The swapfile extent mapping code
rounds the swap extents inwards so that the parts of the on-disk
extents that are not aligned or cannot hold a full page are
omitted from the ranges of the file that can be swapped to.

i.e. a file that extents aligned to 4kB is fine for a 4KB page size
machine, but needs additional alignment to allow swapping to work on
a 64kB page size machine. Hence the swap code rounds the file
extents inwards to PAGE_SIZE to align them correctly. We really
should be doing this for THP page size rather than PAGE_SIZE if
THP_SWAP is enabled, regardless of whether swap clustering is
enabled or not...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx