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

Jan Kara
2025-05-05T10:09:05Z

On Wed 30-04-25 15:59:18, 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>

Looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

> 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.
> + *
> + * Limit to the boundaries of the VMA to avoid reading in any
> + * pad that might exist between sections, which would be a waste
> + * of memory.
> + */
> + struct vm_area_struct *vma = vmf->vma;
> + unsigned long start = vma->vm_pgoff;
> + unsigned long end = start + ((vma->vm_end - vma->vm_start) >> PAGE_SHIFT);
> + unsigned long ra_end;
> +
> + ra->order = exec_folio_order();
> + ra->start = round_down(vmf->pgoff, 1UL << ra->order);
> + ra->start = max(ra->start, start);
> + ra_end = round_up(ra->start + ra->ra_pages, 1UL << ra->order);
> + ra_end = min(ra_end, end);
> + ra->size = ra_end - ra->start;
> + ra->async_size = 0;
> + } else {
> + /*
> + * mmap read-around
> + */
> + 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;
> + }
> ractl._index = ra->start;
> page_cache_ra_order(&ractl, ra);
> return fpin;
> --
> 2.43.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR