Re: [PATCH v2 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free
From: David Hildenbrand
Date: Thu Mar 07 2024 - 07:04:46 EST
On 07.03.24 13:01, Barry Song wrote:
On Thu, Mar 7, 2024 at 7:45 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
On 07.03.24 12:42, Ryan Roberts wrote:
On 07/03/2024 11:31, David Hildenbrand wrote:
On 07.03.24 12:26, Barry Song wrote:
On Thu, Mar 7, 2024 at 7:13 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 07/03/2024 10:54, David Hildenbrand wrote:
On 07.03.24 11:54, David Hildenbrand wrote:
On 07.03.24 11:50, Ryan Roberts wrote:
On 07/03/2024 09:33, Barry Song wrote:
On Thu, Mar 7, 2024 at 10:07 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 07/03/2024 08:10, Barry Song wrote:
On Thu, Mar 7, 2024 at 9:00 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
Hey Barry,
Thanks for taking time to review!
On Thu, Mar 7, 2024 at 3:00 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
On Thu, Mar 7, 2024 at 7:15 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
[...]
+static inline bool can_mark_large_folio_lazyfree(unsigned long addr,
+ struct folio *folio,
pte_t *start_pte)
+{
+ int nr_pages = folio_nr_pages(folio);
+ fpb_t flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+
+ for (int i = 0; i < nr_pages; i++)
+ if (page_mapcount(folio_page(folio, i)) != 1)
+ return false;
we have moved to folio_estimated_sharers though it is not precise, so
we don't do
this check with lots of loops and depending on the subpage's mapcount.
If we don't check the subpage’s mapcount, and there is a cow folio
associated
with this folio and the cow folio has smaller size than this folio,
should we still
mark this folio as lazyfree?
I agree, this is true. However, we've somehow accepted the fact that
folio_likely_mapped_shared
can result in false negatives or false positives to balance the
overhead. So I really don't know :-)
Maybe David and Vishal can give some comments here.
BTW, do we need to rebase our work against David's changes[1]?
[1]
https://lore.kernel.org/linux-mm/20240227201548.857831-1-david@xxxxxxxxxx/
Yes, we should rebase our work against David’s changes.
+
+ return nr_pages == folio_pte_batch(folio, addr, start_pte,
+ ptep_get(start_pte), nr_pages,
flags, NULL);
+}
+
static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
unsigned long end, struct mm_walk
*walk)
@@ -676,11 +690,45 @@ static int madvise_free_pte_range(pmd_t *pmd,
unsigned long addr,
*/
if (folio_test_large(folio)) {
int err;
+ unsigned long next_addr, align;
- if (folio_estimated_sharers(folio) != 1)
- break;
- if (!folio_trylock(folio))
- break;
+ if (folio_estimated_sharers(folio) != 1 ||
+ !folio_trylock(folio))
+ goto skip_large_folio;
I don't think we can skip all the PTEs for nr_pages, as some of them
might be
pointing to other folios.
for example, for a large folio with 16PTEs, you do MADV_DONTNEED(15-16),
and write the memory of PTE15 and PTE16, you get page faults, thus PTE15
and PTE16 will point to two different small folios. We can only skip
when we
are sure nr_pages == folio_pte_batch() is sure.
Agreed. Thanks for pointing that out.
+
+ align = folio_nr_pages(folio) * PAGE_SIZE;
+ next_addr = ALIGN_DOWN(addr + align, align);
+
+ /*
+ * If we mark only the subpages as lazyfree, or
+ * cannot mark the entire large folio as
lazyfree,
+ * then just split it.
+ */
+ if (next_addr > end || next_addr - addr !=
align ||
+ !can_mark_large_folio_lazyfree(addr, folio,
pte))
+ goto split_large_folio;
+
+ /*
+ * Avoid unnecessary folio splitting if the
large
+ * folio is entirely within the given range.
+ */
+ folio_clear_dirty(folio);
+ folio_unlock(folio);
+ for (; addr != next_addr; pte++, addr +=
PAGE_SIZE) {
+ ptent = ptep_get(pte);
+ if (pte_young(ptent) ||
pte_dirty(ptent)) {
+ ptent =
ptep_get_and_clear_full(
+ mm, addr, pte,
tlb->fullmm);
+ ptent = pte_mkold(ptent);
+ ptent = pte_mkclean(ptent);
+ set_pte_at(mm, addr, pte,
ptent);
+ tlb_remove_tlb_entry(tlb, pte,
addr);
+ }
Can we do this in batches? for a CONT-PTE mapped large folio, you are
unfolding
and folding again. It seems quite expensive.
I'm not convinced we should be doing this in batches. We want the initial
folio_pte_batch() to be as loose as possible regarding permissions so
that we
reduce our chances of splitting folios to the min. (e.g. ignore SW bits
like
soft dirty, etc). I think it might be possible that some PTEs are RO and
other
RW too (e.g. due to cow - although with the current cow impl, probably not.
But
its fragile to assume that). Anyway, if we do an initial batch that ignores
all
You are correct. I believe this scenario could indeed occur. For instance,
if process A forks process B and then unmaps itself, leaving B as the
sole process owning the large folio. The current wp_page_reuse() function
will reuse PTE one by one while the specific subpage is written.
Hmm - I thought it would only reuse if the total mapcount for the folio
was 1.
And since it is a large folio with each page mapped once in proc B, I thought
every subpage write would cause a copy except the last one? I haven't
looked at
the code for a while. But I had it in my head that this is an area we need to
improve for mTHP.
So sad I am wrong again 😢
wp_page_reuse() will currently reuse a PTE part of a large folio only if
a single PTE remains mapped (refcount == 0).
^ == 1
seems this needs improvement. it is a waste the last subpage can
My take that is WIP:
https://lore.kernel.org/all/20231124132626.235350-1-david@xxxxxxxxxx/T/#u
reuse the whole large folio. i was doing it in a quite different way,
if the large folio had only one subpage left, i would do copy and
released the large folio[1]. and if i could reuse the whole large folio
with CONT-PTE, i would reuse the whole large folio[2]. in mainline,
we don't have this cont-pte luxury exposed to mm, so i guess we can
not do [2] easily, but [1] seems to be an optimization.
Yeah, I had essentially the same idea: just free up the large folio if most of
the stuff is unmapped. But that's rather a corner-case optimization, so I did
not proceed with that.
I'm not sure it's a corner case, really? - process forks, then both parent and
child and write to all pages in what was previously a fully & contiguously
mapped large folio?
Well, with 2 MiB my assumption was that while it can happen, it's rather
rare. With smaller THP it might get more likely, agreed.
Reggardless, why is it an optimization to do the copy for the last subpage and
syncrhonously free the large folio? It's already partially mapped so is on the
deferred split list and can be split if memory is tight.
we don't want reclamation overhead later. and we want memories immediately
available to others. reclamation will always cause latency and affect User
experience. split_folio is not cheap :-) if the number of this kind of
large folios
is huge, the waste can be huge for some while.
it is not a corner case for large folio swap-in. while someone writes
one subpage, I swap-in a large folio, wp_reuse will immediately
be called. This can cause waste quite often. One outcome of this
discussion is that I realize I should investigate this issue immediately
in the swap-in series as my off-tree code has optimized reuse but
mainline hasn't.
Note that if the swp entry was exclusive, the subpage will be marked
PAE, so wp_reuse() will (and must!) reuse it.
We fallback to the refcount==1 scheme only if PAE is not set for that
subpage.
--
Cheers,
David / dhildenb