On 07.06.24 12:24, Usama Arif wrote:
On 04/06/2024 13:43, David Hildenbrand wrote:
On 04.06.24 14:30, David Hildenbrand wrote:
On 04.06.24 12:58, Usama Arif wrote:
Approximately 10-20% of pages to be swapped out are zero pages [1].
Rather than reading/writing these pages to flash resulting
in increased I/O and flash wear, the pte can be cleared for those
addresses at unmap time while shrinking folio list. When this
causes a page fault, do_pte_missing will take care of this page.
With this patch, NVMe writes in Meta server fleet decreased
by almost 10% with conventional swap setup (zswap disabled).
[1]
https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/
Signed-off-by: Usama Arif <usamaarif642@xxxxxxxxx>
---
include/linux/rmap.h | 1 +
mm/rmap.c | 163
++++++++++++++++++++++---------------------
mm/vmscan.c | 89 ++++++++++++++++-------
3 files changed, 150 insertions(+), 103 deletions(-)
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index bb53e5920b88..b36db1e886e4 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -100,6 +100,7 @@ enum ttu_flags {
* do a final flush if necessary */
TTU_RMAP_LOCKED = 0x80, /* do not grab rmap lock:
* caller holds it */
+ TTU_ZERO_FOLIO = 0x100,/* zero folio */
};
#ifdef CONFIG_MMU
diff --git a/mm/rmap.c b/mm/rmap.c
index 52357d79917c..d98f70876327 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1819,96 +1819,101 @@ static bool try_to_unmap_one(struct folio
*folio, struct vm_area_struct *vma,
*/
dec_mm_counter(mm, mm_counter(folio));
} else if (folio_test_anon(folio)) {
- swp_entry_t entry = page_swap_entry(subpage);
- pte_t swp_pte;
- /*
- * Store the swap location in the pte.
- * See handle_pte_fault() ...
- */
- if (unlikely(folio_test_swapbacked(folio) !=
- folio_test_swapcache(folio))) {
+ if (flags & TTU_ZERO_FOLIO) {
+ pte_clear(mm, address, pvmw.pte);
+ dec_mm_counter(mm, MM_ANONPAGES);
Is there an easy way to reduce the code churn and highlight the added
code?
Like
} else if (folio_test_anon(folio) && (flags & TTU_ZERO_FOLIO)) {
} else if (folio_test_anon(folio)) {
Also to concerns that I want to spell out:
(a) what stops the page from getting modified in the meantime? The CPU
can write it until the TLB was flushed.
Thanks for pointing this out David and Shakeel. This is a big issue in
this v2, and as Shakeel pointed out in [1] we need to do a second rmap
walk. Looking at how ksm deals with this in try_to_merge_one_page which
calls write_protect_page for each vma (i.e. basically an rmap walk),
this would be much more CPU expensive and complicated compared to v1
[2], where the swap subsystem can handle all complexities. I will go
back to my v1 solution for the next revision as its much more simpler
and the memory usage is very low (0.003%) as pointed out by Johannes [3]
which would likely go away with the memory savings of not having a
zswap_entry for zero filled pages, and the solution being a lot simpler
than what a valid v2 approach would look like.
Agreed.
--
Cheers,
David / dhildenb
