On 23/04/2020 10:09, Catalin Marinas wrote:
On Wed, Apr 22, 2020 at 11:08:10AM -0700, Dave Hansen wrote:
On 4/22/20 7:25 AM, Steven Price wrote:
Three new hooks are added to the swap code:
* arch_prepare_to_swap() and
* arch_swap_invalidate_page() / arch_swap_invalidate_area().
One new hook is added to shmem:
* arch_swap_restore_tags()
How do the tags get restored outside of the shmem path? I was expecting
to see more arch_swap_restore_tags() sites.
The restoring is done via set_pte_at() -> mte_sync_tags() ->
mte_restore_tags() in the arch code (see patch 3).
arch_swap_restore_tags() just calls mte_restore_tags() directly.
shmem is slightly problematic as it moves the page from the swap cache
to the shmem one and I think arch_swap_invalidate_page() would have
already been called by the time we get to set_pte_at() (Steven can
correct me if I got this wrong).
That's correct - shmem can pull in pages (into it's own cache) and
invalidate the swap entries without any process having a PTE restored.
So we need to hook shmem to restore the tags even though there's no PTE
restored yet.
The set_pte_at() 'trick' enables delaying the restoring of the tags (in
the usual case) until the I/O for the page has completed, which might be
necessary in some cases if the I/O can clobber the tags in memory. I
couldn't find a better way of hooking this.
Steve
