On 08.09.25 14:25, Lorenzo Stoakes wrote:
On Sat, Sep 06, 2025 at 08:56:48AM +0200, David Hildenbrand wrote:
On 06.09.25 03:05, John Hubbard wrote:
Probably a similar sentiment as Lorenzo here...the above diffs make the code *worse* to read. In fact, I recall adding record_subpages() here long ago, specifically to help clarify what was going on.
Well, there is a lot I dislike about record_subpages() to go back there. Starting with "as Willy keeps explaining, the concept of subpages do not exist and ending with "why do we fill out the array even on failure".
Yes
:)
Now it's been returned to it's original, cryptic form.
The code in the caller was so uncryptic that both me and Lorenzo missed that magical addition. :P
:'(
Just my take on it, for whatever that's worth. :)
As always, appreciated.
I could of course keep the simple loop in some "record_folio_pages" function and clean up what I dislike about record_subpages().
But I much rather want the call chain to be cleaned up instead, if possible.
Roughly, what I am thinking (limiting it to pte+pmd case) about is the following:
I cannot get the below to apply even with the original patch here applied + fix.
It looks like (in mm-new :) commit e73f43a66d5f ("mm/gup: remove dead pgmap refcounting code") by Alastair has conflicted here, but even then I can't make it apply, with/without your fix...!
To be clear: it was never intended to be applied, because it wouldn't even compile in the current form.
It was based on this nth_page submission + fix.
[...]
} static int gup_fast_pud_range(p4d_t *p4dp, p4d_t p4d, unsigned long addr,
OK I guess you intentionally left the rest as a TODO :)
So I'll wait for you to post it before reviewing in-depth.
This generally LGTM as an approach, getting rid of *nr is important that's really horrible.
Yes. Expect a cleanup in that direction soonish (again, either from me or someone else I poke)
-- 2.50.1
Oh, I might even have found a bug moving away from that questionable "ret==1 means success" handling in gup_fast_pte_range()? Will have to double-check, but likely the following is the right thing to do.
From 8f48b25ef93e7ef98611fd58ec89384ad5171782 Mon Sep 17 00:00:00 2001 From: David Hildenbrand david@redhat.com Date: Sat, 6 Sep 2025 08:46:45 +0200 Subject: [PATCH] mm/gup: fix handling of errors from arch_make_folio_accessible() in follow_page_pte()
In case we call arch_make_folio_accessible() and it fails, we would incorrectly return a value that is "!= 0" to the caller, indicating that we pinned all requested pages and that the caller can keep going.
follow_page_pte() is not supposed to return error values, but instead 0 on failure and 1 on success.
That is of course wrong, because the caller will just keep going pinning more pages. If we happen to pin a page afterwards, we're in trouble, because we essentially skipped some pages.
Fixes: f28d43636d6f ("mm/gup/writeback: add callbacks for inaccessible pages") Signed-off-by: David Hildenbrand david@redhat.com
mm/gup.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c index 22420f2069ee1..cff226ec0ee7d 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -2908,8 +2908,7 @@ static int gup_fast_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, * details. */ if (flags & FOLL_PIN) {
ret = arch_make_folio_accessible(folio);if (ret) {
if (arch_make_folio_accessible(folio)) {Oh Lord above. Lol. Yikes.
Yeah I think your fix is valid...
I sent it out earlier today. Fortunately that function shouldn't usually really fail IIUC.