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.
[1]
https://lore.kernel.org/all/nes73bwc5p6yhwt5tw3upxcqrn5kenn6lvqb6exrf4yppmz6jx@ywhuevpkxlvh/
[2]
https://lore.kernel.org/all/20240530102126.357438-1-usamaarif642@xxxxxxxxx/
[3] https://lore.kernel.org/all/20240530122715.GB1222079@xxxxxxxxxxx/
(b) do you properly handle if the page is pinned (or just got pinned)
and we must not discard it?
Oh, and I forgot, are you handling userfaultd as expected? IIRC there
are some really nasty side-effects with userfaultfd even when
userfaultfd is currently not registered for a VMA [1].
[1]
https://lore.kernel.org/linux-mm/3a4b1027-df6e-31b8-b0de-ff202828228d@xxxxxxxxxx/
What should work is replacing all-zero anonymous pages by the shared
zeropage iff the anonymous page is not pinned and we synchronize
against GUP fast. Well, and we handle possible concurrent writes
accordingly.
KSM does essentially that when told to de-duplicate the shared
zeropage, and I was thinking a while ago if we would want a
zeropage-only KSM version that doesn't need stable tress and all that,
but only deduplicates zero-filled pages into the shared zeropage in a
safe way.
Thanks for the pointer to KSM code.
