Re: [syzbot] [mm?] kernel BUG in __vma_reservation_common

[Oscar Salvador <osalvador@suse.com>]

On Mon, May 27, 2024 at 05:50:24AM -0700, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    66ad4829ddd0 Merge tag 'net-6.10-rc1' of git://git.kernel...
> git tree:       net-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c114aa980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=48c05addbb27f3b0
> dashboard link: https://syzkaller.appspot.com/bug?extid=d3fe2dc5ffe9380b714b
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17770d72980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10db1592980000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/05c6f2231ef8/disk-66ad4829.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/5f4fc63b22e3/vmlinux-66ad4829.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/67f5c4c88729/bzImage-66ad4829.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d3fe2dc5ffe9380b714b@syzkaller.appspotmail.com

#syz test

From 917fa54481422c650425c8b0330439f8a3308479 Mon Sep 17 00:00:00 2001
From: Oscar Salvador <osalvador@suse.de>
Date: Tue, 28 May 2024 10:43:14 +0200
Subject: [PATCH] mm/hugetlb: Do not call vma_add_reservation upon ENOMEM

sysbot reported a splat [1] on __unmap_hugepage_range().
This is because vma_needs_reservation() can return -ENOMEM if
allocate_file_region_entries() fails to allocate the file_region struct for
the reservation.
Check for that and do not call vma_add_reservation() if that is the case,
otherwise region_abort() and region_del() will see that we do not have any
file_regions.

If we detect that vma_needs_reservation returned -ENOMEM, we clear the
hugetlb_restore_reserve flag as if this reservation was still consumed,
so free_huge_folio will not increment the resv count.

[1] https://lore.kernel.org/linux-mm/0000000000004096100617c58d54@google.com/T/#ma5983bc1ab18a54910da83416b3f89f3c7ee43aa

Reported-by: syzbot+d3fe2dc5ffe9380b714b@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-mm/0000000000004096100617c58d54@google.com/
Signed-off-by: Oscar Salvador <osalvador@suse.de>
---
 mm/hugetlb.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6be78e7d4f6e..a178e4bcca1b 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5768,8 +5768,20 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
 		 * do_exit() will not see it, and will keep the reservation
 		 * forever.
 		 */
-		if (adjust_reservation && vma_needs_reservation(h, vma, address))
-			vma_add_reservation(h, vma, address);
+		if (adjust_reservation) {
+			int rc = vma_needs_reservation(h, vma, address)
+
+			if (rc < 0)
+				/* Pressumably allocate_file_region_entries failed
+				 * to allocate a file_region struct. Clear
+				 * hugetlb_restore_reserve so that global reserve
+				 * count will not be incremented by free_huge_folio.
+				 * Act as if we consumed the reservation.
+				 */
+				folio_clear_hugetlb_restore_reserve(folio);
+			else if (rc)
+				vma_add_reservation(h, vma, address);
+		}
 
 		tlb_remove_page_size(tlb, page, huge_page_size(h));
 		/*
-- 
2.45.1

-- 
Oscar Salvador
SUSE Labs