+ mm-gup-stop-leaking-pinned-pages-in-low-memory-conditions.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/gup: stop leaking pinned pages in low memory conditions
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-gup-stop-leaking-pinned-pages-in-low-memory-conditions.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-gup-stop-leaking-pinned-pages-in-low-memory-conditions.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: John Hubbard <jhubbard@nvidia.com>
Subject: mm/gup: stop leaking pinned pages in low memory conditions
Date: Wed, 16 Oct 2024 13:22:42 -0700

If a driver tries to call any of the pin_user_pages*(FOLL_LONGTERM) family
of functions, and requests "too many" pages, then the call will
erroneously leave pages pinned.  This is visible in user space as an
actual memory leak.

Repro is trivial: just make enough pin_user_pages(FOLL_LONGTERM) calls to
exhaust memory.

The root cause of the problem is this sequence, within
__gup_longterm_locked():

    __get_user_pages_locked()
    rc = check_and_migrate_movable_pages()

...which gets retried in a loop.  The loop error handling is incomplete,
clearly due to a somewhat unusual and complicated tri-state error API. 
But anyway, if -ENOMEM, or in fact, any unexpected error is returned from
check_and_migrate_movable_pages(), then __gup_longterm_locked() happily
returns the error, while leaving the pages pinned.

In the failed case, which is an app that requests (via a device driver)
30720000000 bytes to be pinned, and then exits, I see this:

    $ grep foll /proc/vmstat
        nr_foll_pin_acquired 7502048
        nr_foll_pin_released 2048

And after applying this patch, it returns to balanced pins:

    $ grep foll /proc/vmstat
        nr_foll_pin_acquired 7502048
        nr_foll_pin_released 7502048

Fix this by unpinning the pages that __get_user_pages_locked() has
pinned, in such error cases.

Link: https://lkml.kernel.org/r/20241016202242.456953-1-jhubbard@nvidia.com
Fixes: 24a95998e9ba ("mm/gup.c: simplify and fix check_and_migrate_movable_pages() return codes")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Shigeru Yoshida <syoshida@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/gup.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/mm/gup.c~mm-gup-stop-leaking-pinned-pages-in-low-memory-conditions
+++ a/mm/gup.c
@@ -2492,6 +2492,17 @@ static long __gup_longterm_locked(struct
 
 		/* FOLL_LONGTERM implies FOLL_PIN */
 		rc = check_and_migrate_movable_pages(nr_pinned_pages, pages);
+
+		/*
+		 * The __get_user_pages_locked() call happens before we know
+		 * that whether it's possible to successfully complete the whole
+		 * operation. To compensate for this, if we get an unexpected
+		 * error (such as -ENOMEM) then we must unpin everything, before
+		 * erroring out.
+		 */
+		if (rc != -EAGAIN && rc != 0)
+			unpin_user_pages(pages, nr_pinned_pages);
+
 	} while (rc == -EAGAIN);
 	memalloc_pin_restore(flags);
 	return rc ? rc : nr_pinned_pages;
_

Patches currently in -mm which might be from jhubbard@nvidia.com are

mm-gup-stop-leaking-pinned-pages-in-low-memory-conditions.patch
kaslr-rename-physmem_end-and-physmem_end-to-direct_map_physmem_end.patch