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

[Rafael Aquini]

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?
> 
> > 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 suspect that will have to come with the 3rd, and final, part of the THP_SWAP
work Intel is doing. Right now, basically, all that's accomplished is deferring 
the THP split step when swapping out, so this change is what we need to
avoid stomping outside the file extent boundaries.


> 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?
>
> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx
>