Re: [PATCH v2 4/4] mm: gup: batch PTE-mapped large folios in gup_fast_pte_range()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 11/2/26 12:30, Yeoreum Yun wrote:
> GUP-fast grabs a PTE-mapped large folio one page at a time.  Every PTE
> costs a try_grab_folio_fast() (a refcount cmpxchg, plus the pincount
> atomic, a full barrier and a node stat update for FOLL_PIN), a
> gup_fast_folio_allowed() and a folio_set_referenced(), so a 64 kB mTHP
> pays sixteen of each and a PTE-mapped 2 MB THP pays 512.
> 
> The users are the hot ones: iov_iter_extract_user_pages() pins every
> O_DIRECT buffer through pin_user_pages_fast(), io_uring registers its
> buffers through it, and so does RDMA memory registration.
> With mTHP those buffers are PTE-mapped large folios.
> 
> After the head page has been grabbed and verified as before, extend the
> run to the following PTEs that map the next pages of the same folio
> with the same protection bits, take the remaining references for the
> run with try_grab_folio(), and fill pages[] from the run.
> The PTEs of the run are read while the reference on the folio is held,
> so the folio can neither be collapsed, freed nor split underneath the scan.
> A read-only run of an anonymous folio stops at the first page
> that gup_must_unshare() rejects, so the per-page exclusivity guarantee for
> FOLL_PIN is kept.
> 
> folio_pte_batch_flags() does the scan.  It reads the entries with
> ptep_get(), which is not safe without the page table lock on
> CONFIG_GUP_GET_PXX_LOW_HIGH (x86 PAE, mips32, sh) or with arm64 contpte,
> and GUP-fast reads every PTE with ptep_get_lockless() for that reason.
> Add FPB_LOCKLESS to make the helper do the same; it is inlined per call
> site, so the other callers do not change.
> 
> The order-0 path keeps its instruction sequence.  The folio flags are
> tested before folio_set_referenced() on purpose: a load of the flags
> right after that locked instruction cost about 25% on order-0 pages in
> the gup_test benchmark below, and reading them first brings that path
> back to the baseline.
> 
> gup_test ioctl over a 256 MB anonymous region, FOLL_WRITE, 65536 pages
> per call, median of 15 runs, two baseline boots / one patched boot, on
> an x86-64 VM:
> 
>                        get_user_pages_fast()   pin_user_pages_fast()
>                        before       after      before       after
>   4 kB pages           849/789 us   783 us     1174/1138 us 1094 us
>   64 kB mTHP           812/811 us   126 us     1215/1113 us  188 us
>   1 MB mTHP            799/774 us    59 us     1185/1077 us   64 us
>   2 MB THP, PTE-mapped 803/786 us    57 us     1159/1120 us   58 us
>   2 MB THP, PMD-mapped  42/40 us     40 us       46/41 us     42 us
> 
> fio O_DIRECT randread of a null_blk device (CPU bound) with the I/O
> buffer in 64 kB mTHP, median of 15 five-second runs:
> 
>   bs=1M psync                 126.2/125.6 GB/s  ->  220.6 GB/s
>   bs=1M io_uring, iodepth 16   71.2/71.6 GB/s   ->  112.1 GB/s
>   bs=64k io_uring, iodepth 16  50.2/50.7 GB/s   ->   60.1 GB/s
> 
> IORING_REGISTER_BUFFERS of 1 GB, median of 15:
> 
>   64 kB mTHP           6179/5998 us  ->  1599 us
>   1 MB mTHP            5790/5801 us  ->   893 us
>   4 kB pages           7173/6741 us  ->  6763 us
> 
> Signed-off-by: Yuan-Hao Hsu <aa9736195201@gmail.com>
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
>  mm/gup.c | 46 +++++++++++++++++++++++++++++-----------------
>  1 file changed, 29 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 49681f63950ec..94a9dc45e5953 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2901,7 +2901,8 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp,
>  		unsigned long addr, unsigned long end,
>  		unsigned int flags, struct page **pages)
>  {
> -	unsigned long nr_pages = 0;
> +	unsigned long nr_pages = 0, nr_batch;
> +	struct folio *folio = NULL;
>  	pte_t *ptep, *ptem;
>  
>  	ptem = ptep = pte_offset_map(&pmd, addr);
> @@ -2910,7 +2911,9 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp,
>  	do {
>  		pte_t pte = ptep_get_lockless(ptep);
>  		struct page *page;
> -		struct folio *folio;
> +		unsigned long i;
> +
> +		nr_batch = 1;
>  
>  		/*
>  		 * Always fallback to ordinary GUP on PROT_NONE-mapped pages:
> @@ -2937,19 +2940,21 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp,
>  			goto pte_unmap;
>  
>  		if (unlikely(pmd_val(pmd) != pmd_val(pmdp_get_lockless(pmdp))) ||
> -		    unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep)))) {
> -			gup_put_folio(folio, 1, flags);
> +		    unlikely(pte_val(pte) != pte_val(ptep_get_lockless(ptep))))
>  			goto pte_unmap;
> -		}

As discussed, we should really add some more details why each of these
comparisons is required. Let's do that in a separate patch.

(I recall the pte check is required for concurrent PTE changes like
unmap/protection etc and the pmd check is for THP collapse on architectures
without IPI during TLB flushes)

>  
> -		if (!gup_fast_folio_allowed(folio, flags)) {
> -			gup_put_folio(folio, 1, flags);
> +		if (!gup_fast_folio_allowed(folio, flags))
>  			goto pte_unmap;
> -		}
>  
> -		if (!pte_write(pte) && gup_must_unshare(NULL, flags, folio, page)) {
> -			gup_put_folio(folio, 1, flags);
> -			goto pte_unmap;
> +		if (folio_test_large(folio))
> +			nr_batch = folio_pte_batch_flags(folio, NULL, ptep, &pte,
> +							 (end - addr) >> PAGE_SHIFT,
> +							 FPB_LOCKLESS | FPB_RESPECT_WRITE);

Batching after the checks above, when we already stabilized the folio by
grabbing a reference should indeed be correct.

Concurrent collapse should already see the raised reference and back off.

> +
> +		if (!pte_write(pte)) {
> +			nr_batch = gup_unshared_pages(NULL, flags, folio, page, nr_batch);
> +			if (nr_batch == 0)

!nr_batch

> +				goto pte_unmap;
>  		}
>  
>  		/*
> @@ -2958,16 +2963,23 @@ static unsigned long gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp,
>  		 * see Documentation/core-api/pin_user_pages.rst for
>  		 * details.
>  		 */
> -		if ((flags & FOLL_PIN) && arch_make_folio_accessible(folio)) {
> -			gup_put_folio(folio, 1, flags);
> +		if ((flags & FOLL_PIN) && arch_make_folio_accessible(folio))
>  			goto pte_unmap;

The label is now slightly misnamed.

> -		}
> +
> +		if (nr_batch > 1 && try_grab_folio(folio, nr_batch - 1, flags))
> +			nr_batch = 1;
> +
>  		folio_set_referenced(folio);
> -		pages[nr_pages] = page;
> -		nr_pages++;
> -	} while (ptep++, addr += PAGE_SIZE, addr != end);
> +		for (i = 0; i < nr_batch; i++) {
> +			pages[nr_pages + i] = page + i;
> +		}

No need for {}. Rik will replace that with a proper helper later.

> +		nr_pages += nr_batch;
> +		folio = NULL;
> +	} while (ptep += nr_batch, addr += nr_batch * PAGE_SIZE, addr != end);
>  
>  pte_unmap:
> +	if (folio)
> +		gup_put_folio(folio, 1, flags);
>  	pte_unmap(ptem);
>  	return nr_pages;
>  }
> 


-- 
Cheers,

David