Re: [RFC PATCH v3 0/8] batch lookups in follow_page_mask()

[Rik van Riel]

On Tue, 2026-08-25 at 21:39 -0700, Christoph Hellwig wrote:
> On Tue, Aug 25, 2026 at 09:37:09AM -0400, Rik van Riel wrote:
> > > And what many including the most performance critical callers
> > > want
> > > instead is really a single bio_vec.  Maybe we can go the extra
> > > step
> > > for
> > > that, as it would reduce the number of calls into gup
> > > significantly.
> > > 
> > Which code are you referring to here?
> > 
> > blk_rq_map_user() seems to end up calling pin_user_pages_fast(),
> > via iov_iter_extract_user_pages, and it directly fills in the
> > bio_vec's pages array.
> > 
> > Is there another performance critical path that goes
> > through the slower get_user_pages() path?
> 
> The other callers of iov_iter_extract_bvecs matter more, but this is
> the main user.
> 
> And iov_iter_extract_bvecs right now is very inefficient when used
> on larger folios,

I think it would be possible to create a path
into follow_page_mask() that passes a bio_vec,
and where we fill in the bio_vec fields the
same way bvec_set_page() does.

Then iov_iter_extract_bvecs would no longer
need to iterate over the pages returned by
get_user_pages_fast.

That seems like a follow-up series though,
and we need to think carefully about whether
we would also want to unify part of the 
gup_fast and gup code.

Currently the gup code depends on locking
to keep page tables from going away, while
the gup_fast code depends on disabling irqs.

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