Re: [PATCH v3 2/3] vmalloc: Optimize vfree

[Uladzislau Rezki]

On Tue, Mar 24, 2026 at 10:55:55AM -0400, Zi Yan wrote:
> On 24 Mar 2026, at 9:35, Muhammad Usama Anjum wrote:
> 
> > From: Ryan Roberts <ryan.roberts@xxxxxxx>
> >
> > Whenever vmalloc allocates high order pages (e.g. for a huge mapping) it
> > must immediately split_page() to order-0 so that it remains compatible
> > with users that want to access the underlying struct page.
> > Commit a06157804399 ("mm/vmalloc: request large order pages from buddy
> > allocator") recently made it much more likely for vmalloc to allocate
> > high order pages which are subsequently split to order-0.
> >
> > Unfortunately this had the side effect of causing performance
> > regressions for tight vmalloc/vfree loops (e.g. test_vmalloc.ko
> > benchmarks). See Closes: tag. This happens because the high order pages
> > must be gotten from the buddy but then because they are split to
> > order-0, when they are freed they are freed to the order-0 pcp.
> > Previously allocation was for order-0 pages so they were recycled from
> > the pcp.
> >
> > It would be preferable if when vmalloc allocates an (e.g.) order-3 page
> > that it also frees that order-3 page to the order-3 pcp, then the
> > regression could be removed.
> >
> > So let's do exactly that; use the new __free_contig_range() API to
> > batch-free contiguous ranges of pfns. This not only removes the
> > regression, but significantly improves performance of vfree beyond the
> > baseline.
> >
> > A selection of test_vmalloc benchmarks running on arm64 server class
> > system. mm-new is the baseline. Commit a06157804399 ("mm/vmalloc: request
> > large order pages from buddy allocator") was added in v6.19-rc1 where we
> > see regressions. Then with this change performance is much better. (>0
> > is faster, <0 is slower, (R)/(I) = statistically significant
> > Regression/Improvement):
> >
> > +-----------------+----------------------------------------------------------+-------------------+--------------------+
> > | Benchmark       | Result Class                                             |   mm-new          |  this series       |
> > +=================+==========================================================+===================+====================+
> > | micromm/vmalloc | fix_align_alloc_test: p:1, h:0, l:500000 (usec)          |        1331843.33 |         (I) 67.17% |
> > |                 | fix_size_alloc_test: p:1, h:0, l:500000 (usec)           |         415907.33 |             -5.14% |
> > |                 | fix_size_alloc_test: p:4, h:0, l:500000 (usec)           |         755448.00 |         (I) 53.55% |
> > |                 | fix_size_alloc_test: p:16, h:0, l:500000 (usec)          |        1591331.33 |         (I) 57.26% |
> > |                 | fix_size_alloc_test: p:16, h:1, l:500000 (usec)          |        1594345.67 |         (I) 68.46% |
> > |                 | fix_size_alloc_test: p:64, h:0, l:100000 (usec)          |        1071826.00 |         (I) 79.27% |
> > |                 | fix_size_alloc_test: p:64, h:1, l:100000 (usec)          |        1018385.00 |         (I) 84.17% |
> > |                 | fix_size_alloc_test: p:256, h:0, l:100000 (usec)         |        3970899.67 |         (I) 77.01% |
> > |                 | fix_size_alloc_test: p:256, h:1, l:100000 (usec)         |        3821788.67 |         (I) 89.44% |
> > |                 | fix_size_alloc_test: p:512, h:0, l:100000 (usec)         |        7795968.00 |         (I) 82.67% |
> > |                 | fix_size_alloc_test: p:512, h:1, l:100000 (usec)         |        6530169.67 |        (I) 118.09% |
> > |                 | full_fit_alloc_test: p:1, h:0, l:500000 (usec)           |         626808.33 |             -0.98% |
> > |                 | kvfree_rcu_1_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |         532145.67 |             -1.68% |
> > |                 | kvfree_rcu_2_arg_vmalloc_test: p:1, h:0, l:500000 (usec) |         537032.67 |             -0.96% |
> > |                 | long_busy_list_alloc_test: p:1, h:0, l:500000 (usec)     |        8805069.00 |         (I) 74.58% |
> > |                 | pcpu_alloc_test: p:1, h:0, l:500000 (usec)               |         500824.67 |              4.35% |
> > |                 | random_size_align_alloc_test: p:1, h:0, l:500000 (usec)  |        1637554.67 |         (I) 76.99% |
> > |                 | random_size_alloc_test: p:1, h:0, l:500000 (usec)        |        4556288.67 |         (I) 72.23% |
> > |                 | vm_map_ram_test: p:1, h:0, l:500000 (usec)               |         107371.00 |             -0.70% |
> > +-----------------+----------------------------------------------------------+-------------------+--------------------+
> >
> > Fixes: a06157804399 ("mm/vmalloc: request large order pages from buddy allocator")
> > Closes: https://lore.kernel.org/all/66919a28-bc81-49c9-b68f-dd7c73395a0d@xxxxxxx/
> > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Co-developed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> > Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxx>
> > ---
> > Changes since v2:
> > - Remove BUG_ON in favour of simple implementation as this has never
> >   been seen to output any bug in the past as well
> > - Move the free loop to separate function, free_pages_bulk()
> > - Update stats, lruvec_stat in separate loop
> >
> > Changes since v1:
> > - Rebase on mm-new
> > - Rerun benchmarks
> >
> > Made-with: Cursor
> > ---
> >  include/linux/gfp.h |  2 ++
> >  mm/page_alloc.c     | 23 +++++++++++++++++++++++
> >  mm/vmalloc.c        | 16 +++++-----------
> >  3 files changed, 30 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index 7c1f9da7c8e56..71f9097ab99a0 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -239,6 +239,8 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
> >  				struct page **page_array);
> >  #define __alloc_pages_bulk(...)			alloc_hooks(alloc_pages_bulk_noprof(__VA_ARGS__))
> >
> > +void free_pages_bulk(struct page **page_array, unsigned long nr_pages);
> > +
> >  unsigned long alloc_pages_bulk_mempolicy_noprof(gfp_t gfp,
> >  				unsigned long nr_pages,
> >  				struct page **page_array);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index eedce9a30eb7e..250cc07e547b8 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5175,6 +5175,29 @@ unsigned long alloc_pages_bulk_noprof(gfp_t gfp, int preferred_nid,
> >  }
> >  EXPORT_SYMBOL_GPL(alloc_pages_bulk_noprof);
> >
> > +void free_pages_bulk(struct page **page_array, unsigned long nr_pages)
> > +{
> > +	unsigned long start_pfn = 0, pfn;
> > +	unsigned long i, nr_contig = 0;
> > +
> > +	for (i = 0; i < nr_pages; i++) {
> > +		pfn = page_to_pfn(page_array[i]);
> > +		if (!nr_contig) {
> > +			start_pfn = pfn;
> > +			nr_contig = 1;
> > +		} else if (start_pfn + nr_contig != pfn) {
> > +			__free_contig_range(start_pfn, nr_contig);
> > +			start_pfn = pfn;
> > +			nr_contig = 1;
> > +			cond_resched();
>
It will cause schedule while atomic. Have you checked that
__free_contig_range() also can sleep? Of so then we are aligned, if not
probably we should remove it.

--
Uladzislau Rezki