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SubjectRe: [PATCH 6/7] mm/gup: Accelerate thp gup even for "pages != NULL"
On Mon, Jun 19, 2023 at 03:37:30PM -0400, Peter Xu wrote:
> Here what I can do is still guard this try_grab_folio() and fail the GUP if
> for any reason it failed. Perhaps then it means I'll also keep that one
> untouched in hugetlb_follow_page_mask() too. But I suppose keeping the
> WARN_ON_ONCE() seems still proper.

Here's the outcome that I plan to post in the new version, taking care of
try_grab_folio() failures even if it happens, meanwhile remove the
compound_head() redundancy on the page.

__get_user_pages():
...
===8<===
/*
* This must be a large folio (and doesn't need to
* be the whole folio; it can be part of it), do
* the refcount work for all the subpages too.
*
* NOTE: here the page may not be the head page
* e.g. when start addr is not thp-size aligned.
* try_grab_folio() should have taken care of tail
* pages.
*/
if (page_increm > 1) {
struct folio *folio;

/*
* Since we already hold refcount on the
* large folio, this should never fail.
*/
folio = try_grab_folio(page, page_increm - 1,
foll_flags);
if (WARN_ON_ONCE(!folio)) {
/*
* Release the 1st page ref if the
* folio is problematic, fail hard.
*/
gup_put_folio(page_folio(page), 1,
foll_flags);
ret = -EFAULT;
goto out;
}
}
===8<===

Thanks,

--
Peter Xu

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