Re: [RFC PATCH v4 5/5] mm/filemap: Allow arch to request folio size for exec memory

Will Deacon
2025-05-09T15:18:09Z

On Wed, Apr 30, 2025 at 03:59:18PM +0100, Ryan Roberts wrote:
> Change the readahead config so that if it is being requested for an
> executable mapping, do a synchronous read into a set of folios with an
> arch-specified order and in a naturally aligned manner. We no longer
> center the read on the faulting page but simply align it down to the
> previous natural boundary. Additionally, we don't bother with an
> asynchronous part.
>
> On arm64 if memory is physically contiguous and naturally aligned to the
> "contpte" size, we can use contpte mappings, which improves utilization
> of the TLB. When paired with the "multi-size THP" feature, this works
> well to reduce dTLB pressure. However iTLB pressure is still high due to
> executable mappings having a low likelihood of being in the required
> folio size and mapping alignment, even when the filesystem supports
> readahead into large folios (e.g. XFS).
>
> The reason for the low likelihood is that the current readahead
> algorithm starts with an order-0 folio and increases the folio order by
> 2 every time the readahead mark is hit. But most executable memory tends
> to be accessed randomly and so the readahead mark is rarely hit and most
> executable folios remain order-0.
>
> So let's special-case the read(ahead) logic for executable mappings. The
> trade-off is performance improvement (due to more efficient storage of
> the translations in iTLB) vs potential for making reclaim more difficult
> (due to the folios being larger so if a part of the folio is hot the
> whole thing is considered hot). But executable memory is a small portion
> of the overall system memory so I doubt this will even register from a
> reclaim perspective.
>
> I've chosen 64K folio size for arm64 which benefits both the 4K and 16K
> base page size configs. Crucially the same amount of data is still read
> (usually 128K) so I'm not expecting any read amplification issues. I
> don't anticipate any write amplification because text is always RO.
>
> Note that the text region of an ELF file could be populated into the
> page cache for other reasons than taking a fault in a mmapped area. The
> most common case is due to the loader read()ing the header which can be
> shared with the beginning of text. So some text will still remain in
> small folios, but this simple, best effort change provides good
> performance improvements as is.
>
> Confine this special-case approach to the bounds of the VMA. This
> prevents wasting memory for any padding that might exist in the file
> between sections. Previously the padding would have been contained in
> order-0 folios and would be easy to reclaim. But now it would be part of
> a larger folio so more difficult to reclaim. Solve this by simply not
> reading it into memory in the first place.
>
> Benchmarking
> ============
> TODO: NUMBERS ARE FOR V3 OF SERIES. NEED TO RERUN FOR THIS VERSION.
>
> The below shows nginx and redis benchmarks on Ampere Altra arm64 system.
>
> First, confirmation that this patch causes more text to be contained in
> 64K folios:
>
> | File-backed folios | system boot | nginx | redis |
> | by size as percentage |-----------------|-----------------|-----------------|
> | of all mapped text mem | before | after | before | after | before | after |
> |========================|========|========|========|========|========|========|
> | base-page-4kB | 26% | 9% | 27% | 6% | 21% | 5% |
> | thp-aligned-8kB | 4% | 2% | 3% | 0% | 4% | 1% |
> | thp-aligned-16kB | 57% | 21% | 57% | 6% | 54% | 10% |
> | thp-aligned-32kB | 4% | 1% | 4% | 1% | 3% | 1% |
> | thp-aligned-64kB | 7% | 65% | 8% | 85% | 9% | 72% |
> | thp-aligned-2048kB | 0% | 0% | 0% | 0% | 7% | 8% |
> | thp-unaligned-16kB | 1% | 1% | 1% | 1% | 1% | 1% |
> | thp-unaligned-32kB | 0% | 0% | 0% | 0% | 0% | 0% |
> | thp-unaligned-64kB | 0% | 0% | 0% | 1% | 0% | 1% |
> | thp-partial | 1% | 1% | 0% | 0% | 1% | 1% |
> |------------------------|--------|--------|--------|--------|--------|--------|
> | cont-aligned-64kB | 7% | 65% | 8% | 85% | 16% | 80% |
>
> The above shows that for both workloads (each isolated with cgroups) as
> well as the general system state after boot, the amount of text backed
> by 4K and 16K folios reduces and the amount backed by 64K folios
> increases significantly. And the amount of text that is contpte-mapped
> significantly increases (see last row).
>
> And this is reflected in performance improvement:
>
> | Benchmark | Improvement |
> +===============================================+======================+
> | pts/nginx (200 connections) | 8.96% |
> | pts/nginx (1000 connections) | 6.80% |
> +-----------------------------------------------+----------------------+
> | pts/redis (LPOP, 50 connections) | 5.07% |
> | pts/redis (LPUSH, 50 connections) | 3.68% |
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> arch/arm64/include/asm/pgtable.h | 8 +++++++
> include/linux/pgtable.h | 11 +++++++++
> mm/filemap.c | 40 ++++++++++++++++++++++++++------
> 3 files changed, 52 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 2a77f11b78d5..9eb35af0d3cf 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -1537,6 +1537,14 @@ static inline void update_mmu_cache_range(struct vm_fault *vmf,
> */
> #define arch_wants_old_prefaulted_pte cpu_has_hw_af
>
> +/*
> + * Request exec memory is read into pagecache in at least 64K folios. This size
> + * can be contpte-mapped when 4K base pages are in use (16 pages into 1 iTLB
> + * entry), and HPA can coalesce it (4 pages into 1 TLB entry) when 16K base
> + * pages are in use.
> + */
> +#define exec_folio_order() ilog2(SZ_64K >> PAGE_SHIFT)
> +
> static inline bool pud_sect_supported(void)
> {
> return PAGE_SIZE == SZ_4K;
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index b50447ef1c92..1dd539c49f90 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -456,6 +456,17 @@ static inline bool arch_has_hw_pte_young(void)
> }
> #endif
>
> +#ifndef exec_folio_order
> +/*
> + * Returns preferred minimum folio order for executable file-backed memory. Must
> + * be in range [0, PMD_ORDER). Default to order-0.
> + */
> +static inline unsigned int exec_folio_order(void)
> +{
> + return 0;
> +}
> +#endif
> +
> #ifndef arch_check_zapped_pte
> static inline void arch_check_zapped_pte(struct vm_area_struct *vma,
> pte_t pte)
> diff --git a/mm/filemap.c b/mm/filemap.c
> index e61f374068d4..37fe4a55c00d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3252,14 +3252,40 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
> if (mmap_miss > MMAP_LOTSAMISS)
> return fpin;
>
> - /*
> - * mmap read-around
> - */
> fpin = maybe_unlock_mmap_for_io(vmf, fpin);
> - ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
> - ra->size = ra->ra_pages;
> - ra->async_size = ra->ra_pages / 4;
> - ra->order = 0;
> + if (vm_flags & VM_EXEC) {
> + /*
> + * Allow arch to request a preferred minimum folio order for
> + * executable memory. This can often be beneficial to
> + * performance if (e.g.) arm64 can contpte-map the folio.
> + * Executable memory rarely benefits from readahead, due to its
> + * random access nature, so set async_size to 0.

In light of this observation (about randomness of instruction fetch), do
you think it's worth ignoring VM_RAND_READ for VM_EXEC?

Either way, I was looking at this because it touches arm64 and it looks
fine to me:

Acked-by: Will Deacon <will@kernel.org>

Will