Re: [RFC PATCH v1] mm/filemap: Allow arch to request folio size for exec memory

[Ryan Roberts]

On 12/01/2024 10:13, Barry Song wrote:
> On Fri, Jan 12, 2024 at 4:41 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>>
>> Change the readahead config so that if it is being requested for an
>> executable mapping, do a synchronous read of an arch-specified size in a
>> naturally aligned manner.
>>
>> 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" changes, this works
>> well to reduce dTLB pressure. However iTLB pressure is still high due to
>> executable mappings having a low liklihood 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 liklihood is that the current readahead algorithm
>> starts with an order-2 folio and increases the folio order by 2 every
>> time the readahead mark is hit. But most executable memory is faulted in
>> fairly randomly and so the readahead mark is rarely hit and most
>> executable folios remain order-2. This is observed impirically and
>> confirmed from discussion with a gnu linker expert; in general, the
>> linker does nothing to group temporally accessed text together
>> spacially. Additionally, with the current read-around approach there are
>> no alignment guarrantees between the file and folio. This is
>> insufficient for arm64's contpte mapping requirement (order-4 for 4K
>> base pages).
>>
>> So it seems reasonable to 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 read
>> amplification (due to reading too much data around the fault which won't
>> be used), and the latter is independent of base page size. I've chosen
>> 64K folio size for arm64 which benefits both the 4K and 16K base page
>> size configs and shouldn't lead to any further read-amplification since
>> the old read-around path was (usually) reading blocks of 128K (with the
>> last 32K being async).
>>
>> Performance Benchmarking
>> ------------------------
>>
>> The below shows kernel compilation and speedometer javascript benchmarks
>> on Ampere Altra arm64 system. (The contpte patch series is applied in
>> the baseline).
>>
>> First, confirmation that this patch causes more memory to be contained
>> in 64K folios (this is for all file-backed memory so includes
>> non-executable too):
>>
>> | File-backed folios      |   Speedometer   |  Kernel Compile |
>> | by size as percentage   |-----------------|-----------------|
>> | of all mapped file mem  | before |  after | before |  after |
>> |=========================|========|========|========|========|
>> |file-thp-aligned-16kB    |    45% |     9% |    46% |     7% |
>> |file-thp-aligned-32kB    |     2% |     0% |     3% |     1% |
>> |file-thp-aligned-64kB    |     3% |    63% |     5% |    80% |
>> |file-thp-aligned-128kB   |    11% |    11% |     0% |     0% |
>> |file-thp-unaligned-16kB  |     1% |     0% |     3% |     1% |
>> |file-thp-unaligned-128kB |     1% |     0% |     0% |     0% |
>> |file-thp-partial         |     0% |     0% |     0% |     0% |
>> |-------------------------|--------|--------|--------|--------|
>> |file-cont-aligned-64kB   |    16% |    75% |     5% |    80% |
>>
>> The above shows that for both use cases, the amount of file memory
>> backed by 16K folios reduces and the amount backed by 64K folios
>> increases significantly. And the amount of memory that is contpte-mapped
>> significantly increases (last line).
>>
>> And this is reflected in performance improvement:
>>
>> Kernel Compilation (smaller is faster):
>> | kernel   |   real-time |   kern-time |   user-time |   peak memory |
>> |----------|-------------|-------------|-------------|---------------|
>> | before   |        0.0% |        0.0% |        0.0% |          0.0% |
>> | after    |       -1.6% |       -2.1% |       -1.7% |          0.0% |
>>
>> Speedometer (bigger is faster):
>> | kernel   |   runs_per_min |   peak memory |
>> |----------|----------------|---------------|
>> | before   |           0.0% |          0.0% |
>> | after    |           1.3% |          1.0% |
>>
>> Both benchmarks show a ~1.5% improvement once the patch is applied.
>>
>> Alternatives
>> ------------
>>
>> I considered (and rejected for now - but I anticipate this patch will
>> stimulate discussion around what the best approach is) alternative
>> approaches:
>>
>>   - Expose a global user-controlled knob to set the preferred folio
>>     size; this would move policy to user space and allow (e.g.) setting
>>     it to PMD-size for even better iTLB utilizaiton. But this would add
>>     ABI, and I prefer to start with the simplest approach first. It also
>>     has the downside that a change wouldn't apply to memory already in
>>     the page cache that is in active use (e.g. libc) so we don't get the
>>     same level of utilization as for something that is fixed from boot.
>>
>>   - Add a per-vma attribute to allow user space to specify preferred
>>     folio size for memory faulted from the range. (we've talked about
>>     such a control in the context of mTHP). The dynamic loader would
>>     then be responsible for adding the annotations. Again this feels
>>     like something that could be added later if value was demonstrated.
>>
>>   - Enhance MADV_COLLAPSE to collapse to THP sizes less than PMD-size.
>>     This would still require dynamic linker involvement, but would
>>     additionally neccessitate a copy and all memory in the range would
>>     be synchronously faulted in, adding to application load time. It
>>     would work for filesystems that don't support large folios though.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>
>> Hi all,
>>
>> I originally concocted something similar to this, with Matthew's help, as a
>> quick proof of concept hack. Since then I've tried a few different approaches
>> but always came back to this as the simplest solution. I expect this will raise
>> a few eyebrows but given it is providing a real performance win, I hope we can
>> converge to something that can be upstreamed.
>>
>> This depends on my contpte series to actually set the contiguous bit in the page
>> table.
>>
>> Thanks,
>> Ryan
>>
>>
>>  arch/arm64/include/asm/pgtable.h | 12 ++++++++++++
>>  include/linux/pgtable.h          | 12 ++++++++++++
>>  mm/filemap.c                     | 19 +++++++++++++++++++
>>  3 files changed, 43 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index f5bf059291c3..8f8f3f7eb8d8 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -1143,6 +1143,18 @@ 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. The
>> + * trade-off here is performance improvement due to storing translations more
>> + * effciently in the iTLB vs the potential for read amplification due to reading
>> + * data from disk that won't be used. The latter is independent of base page
>> + * size, so we set a page-size independent block size of 64K. 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 arch_wants_exec_folio_order(void) 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 170925379534..57090616d09c 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -428,6 +428,18 @@ static inline bool arch_has_hw_pte_young(void)
>>  }
>>  #endif
>>
>> +#ifndef arch_wants_exec_folio_order
>> +/*
>> + * Returns preferred minimum folio order for executable file-backed memory. Must
>> + * be in range [0, PMD_ORDER]. Negative value implies that the HW has no
>> + * preference and mm will not special-case executable memory in the pagecache.
>> + */
>> +static inline int arch_wants_exec_folio_order(void)
>> +{
>> +       return -1;
>> +}
>> +#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 67ba56ecdd32..80a76d755534 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3115,6 +3115,25 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>>         }
>>  #endif
>>
>> +       /*
>> +        * 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
>> +        * read-ahead anyway, due to its random access nature.
>> +        */
>> +       if (vm_flags & VM_EXEC) {
>> +               int order = arch_wants_exec_folio_order();
>> +
>> +               if (order >= 0) {
>> +                       fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>> +                       ra->size = 1UL << order;
>> +                       ra->async_size = 0;
>> +                       ractl._index &= ~((unsigned long)ra->size - 1);
>> +                       page_cache_ra_order(&ractl, ra, order);
>> +                       return fpin;
>> +               }
>> +       }
> 
> I don't know, but most filesystems don't support large mapping,even iomap.

True, but more are coming. For example ext4 is in the works:
https://lore.kernel.org/all/20240102123918.799062-1-yi.zhang@xxxxxxxxxxxxxxx/

> This patch might negatively affect them. i feel we need to check
> mapping_large_folio_support() at least.

page_cache_ra_order() does this check and falls back to small folios if needed.
So correctness-wise it all works out. I guess your concern is performance due to
effectively removing the async readahead aspect? But if that is a problem, then
it's not just a problem if we are reading small folios, so I don't think the
proposed check is correct.

Perhaps an alternative would be to double ra->size and set ra->async_size to
(ra->size / 2)? That would ensure we always have 64K aligned blocks but would
give us an async portion so readahead can still happen.

I don't feel very expert with this area of the code so I might be talking
rubbish - would be great to hear from others.

> 
>> +
>>         /* If we don't want any read-ahead, don't bother */
>>         if (vm_flags & VM_RAND_READ)
>>                 return fpin;
>> --
>> 2.25.1
> 
> Thanks
> Barry