linux-kernel - [PATCH v2 14/32] mm/userfaultfd: retry if pte_offset

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Date:   Thu, 8 Jun 2023 18:24:38 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Ryan Roberts <ryan.roberts@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v2 14/32] mm/userfaultfd: retry if pte_offset_map() fails

Instead of worrying whether the pmd is stable, userfaultfd_must_wait()
call pte_offset_map() as before, but go back to try again if that fails.

Risk of endless loop?  It already broke out if pmd_none(), !pmd_present()
or pmd_trans_huge(), and pte_offset_map() would have cleared pmd_bad():
which leaves pmd_devmap().  Presumably pmd_devmap() is inappropriate in
a vma subject to userfaultfd (it would have been mistreated before),
but add a check just to avoid all possibility of endless loop there.

Signed-off-by: Hugh Dickins <hughd@...gle.com>
Acked-by: Peter Xu <peterx@...hat.com>
---
 fs/userfaultfd.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index f7a0817b1ec0..ca83423f8d54 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -349,12 +349,13 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 	if (!pud_present(*pud))
 		goto out;
 	pmd = pmd_offset(pud, address);
+again:
 	_pmd = pmdp_get_lockless(pmd);
 	if (pmd_none(_pmd))
 		goto out;
 
 	ret = false;
-	if (!pmd_present(_pmd))
+	if (!pmd_present(_pmd) || pmd_devmap(_pmd))
 		goto out;
 
 	if (pmd_trans_huge(_pmd)) {
@@ -363,11 +364,11 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 		goto out;
 	}
 
-	/*
-	 * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
-	 * and use the standard pte_offset_map() instead of parsing _pmd.
-	 */
 	pte = pte_offset_map(pmd, address);
+	if (!pte) {
+		ret = true;
+		goto again;
+	}
 	/*
 	 * Lockless access: we're in a wait_event so it's ok if it
 	 * changes under us.  PTE markers should be handled the same as none
-- 
2.35.3