Re: [PATCH v3 6/6] mm: gup: batch PTE-mapped large folios in gup_fast_pte_range()

[Yeoreum Yun]

On Tue, Nov 10, 2026 at 02:09:32PM +0100, David Hildenbrand (Arm) wrote:
> On 11/9/26 13:32, 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@xxxxxxxxx>
> 
> There should likely be a co-developed-by? (also in the other one?)

Okay. I'll change.

> 
> LGTM, thanks.
> 
> Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

Thanks!
> 
> -- 
> Cheers,
> 
> David

-- 
Sincerely,
Yeoreum Yun