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console output: https://syzkaller.appspot.com/x/log.txt?x=1150f670580000 >> kernel config:  https://syzkaller.appspot.com/x/.config?x=5929ac65be9baf3c >> dashboard link: https://syzkaller.appspot.com/bug?extid=2b99589e33edbe9475ca >> compiler:       Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2 >> >> Unfortunately, I don't have any reproducer for this issue yet. >> >> Downloadable assets: >> disk image: https://storage.googleapis.com/syzbot-assets/0a42ae72fe0e/disk-627277ba.raw.xz >> vmlinux: https://storage.googleapis.com/syzbot-assets/0be88297bb66/vmlinux-627277ba.xz >> kernel image: https://storage.googleapis.com/syzbot-assets/31808a4b1210/bzImage-627277ba.xz >> >> IMPORTANT: if you fix the issue, please add the following tag to the commit: >> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com >> >> ------------[ cut here ]------------ >> WARNING: CPU: 1 PID: 38 at ./include/linux/mm.h:1335 folio_large_mapcount+0xd0/0x110 include/linux/mm.h:1335 > > This should be > > VM_WARN_ON_FOLIO(!folio_test_large(folio), folio); > >> Modules linked in: >> CPU: 1 UID: 0 PID: 38 Comm: khugepaged Not tainted 6.15.0-rc6-syzkaller-00025-g627277ba7c23 #0 PREEMPT(full) >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025 >> RIP: 0010:folio_large_mapcount+0xd0/0x110 include/linux/mm.h:1335 >> Code: 04 38 84 c0 75 29 8b 03 ff c0 5b 41 5e 41 5f e9 96 d2 2b 09 cc e8 d0 cb 99 ff 48 89 df 48 c7 c6 20 de 77 8b e8 a1 dc de ff 90 <0f> 0b 90 eb b6 89 d9 80 e1 07 80 c1 03 38 c1 7c cb 48 89 df e8 87 >> RSP: 0018:ffffc90000af77e0 EFLAGS: 00010246 >> RAX: e1fcb38c0ff8ce00 RBX: ffffea00014c8000 RCX: e1fcb38c0ff8ce00 >> RDX: 0000000000000001 RSI: ffffffff8d9226df RDI: ffff88801e2fbc00 >> RBP: ffffc90000af7b50 R08: ffff8880b8923e93 R09: 1ffff110171247d2 >> R10: dffffc0000000000 R11: ffffed10171247d3 R12: 1ffffd4000299000 >> R13: dffffc0000000000 R14: 0000000000000000 R15: dffffc0000000000 >> FS:  0000000000000000(0000) GS:ffff8881261fb000(0000) knlGS:0000000000000000 >> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >> CR2: 00007ffe58f12dc0 CR3: 0000000030e04000 CR4: 00000000003526f0 >> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >> Call Trace: >>   >>   folio_mapcount include/linux/mm.h:1369 [inline] > > And here we come through > > if (likely(!folio_test_large(folio))) { >     ... > } > return folio_large_mapcount(folio); > > > So the folio is split concurrently. And I think there is nothing stopping it from getting freed. > > We do a xas_for_each() under RCU. So yes, this is racy. > > In  collapse_file(), we re-validate everything. > > We could > > (A) Take proper pagecache locks > > (B) Try grabbing a temporary folio reference > > (C) Try snapshotting the folio > > Probably, in this code, (B) might be cleanest for now? Handling it just like other code in mm/filemap.c. > Hi, I've implemented your suggestion (B) using folio_try_get(). Could you please review if my patch looks correct? Tested it using existing selftests: sudo make -C tools/testing/selftests/mm run_tests Other two instances of is_refcount_suitable() uses folio locking. Should we maintain consistency with those? Thanks, Shivank #syz test --------------GXQVz0EIt8DBlOQ1kU3p0V3K Content-Type: text/plain; charset=UTF-8; name="0001-mm-khugepaged-Fix-race-with-folio-splitting-in-hpage.patch" Content-Disposition: attachment; filename*0="0001-mm-khugepaged-Fix-race-with-folio-splitting-in-hpage.pa"; filename*1="tch" Content-Transfer-Encoding: base64 RnJvbSBkMWMzNDI3ZTgwMjE1ZmVhOTkyNDI4YzhiNWNhZjUyOTE3MjVkZDY1IE1vbiBTZXAgMTcg MDA6MDA6MDAgMjAwMQpGcm9tOiBTaGl2YW5rIEdhcmcgPHNoaXZhbmtnQGFtZC5jb20+CkRhdGU6 IE1vbiwgMTkgTWF5IDIwMjUgMjA6MTk6MzIgKzAwMDAKU3ViamVjdDogW1BBVENIXSBtbS9raHVn ZXBhZ2VkOiBGaXggcmFjZSB3aXRoIGZvbGlvIHNwbGl0dGluZyBpbgogaHBhZ2VfY29sbGFwc2Vf c2Nhbl9maWxlKCkKCmZvbGlvX21hcGNvdW50KCkgY2hlY2tzIGZvbGlvX3Rlc3RfbGFyZ2UoKSBi ZWZvcmUgcHJvY2VlZGluZyB0bwpmb2xpb19sYXJnZV9tYXBjb3VudCgpLCBidXQgdGhlcmUgZXhp c3RzIGEgcmFjZSB3aW5kb3cgd2hlcmUgYSBmb2xpbwpjb3VsZCBiZSBzcGxpdCBiZXR3ZWVuIHRo ZXNlIGNoZWNrcyB3aGljaCB0cmlnZ2VyZWQgdGhlClZNX1dBUk5fT05fRk9MSU8oIWZvbGlvX3Rl c3RfbGFyZ2UoZm9saW8pLCBmb2xpbykgaW4KZm9saW9fbGFyZ2VfbWFwY291bnQoKS4KClRha2Ug YSB0ZW1wb3JhcnkgZm9saW8gcmVmZXJlbmNlIGluIGhwYWdlX2NvbGxhcHNlX3NjYW5fZmlsZSgp IHRvIHByZXZlbnQKcmFjZXMgd2l0aCBjb25jdXJyZW50IGZvbGlvIHNwbGl0dGluZy9mcmVlaW5n LiBUaGlzIHByZXZlbnQgcG90ZW50aWFsCmluY29ycmVjdCBsYXJnZSBmb2xpbyBkZXRlY3Rpb24u CgpSZXBvcnRlZC1ieTogc3l6Ym90KzJiOTk1ODllMzNlZGJlOTQ3NWNhQHN5emthbGxlci5hcHBz cG90bWFpbC5jb20KQ2xvc2VzOiBodHRwczovL2xvcmUua2VybmVsLm9yZy9hbGwvNjgyODQ3MGQu YTcwYTAyMjAuMzhmMjU1LjAwMGMuR0FFQGdvb2dsZS5jb20KU3VnZ2VzdGVkLWJ5OiBEYXZpZCBI aWxkZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT4KU2lnbmVkLW9mZi1ieTogU2hpdmFuayBHYXJn IDxzaGl2YW5rZ0BhbWQuY29tPgotLS0KIG1tL2todWdlcGFnZWQuYyB8IDE4ICsrKysrKysrKysr KysrKysrKwogMSBmaWxlIGNoYW5nZWQsIDE4IGluc2VydGlvbnMoKykKCmRpZmYgLS1naXQgYS9t bS9raHVnZXBhZ2VkLmMgYi9tbS9raHVnZXBhZ2VkLmMKaW5kZXggY2M5NDVjNmFiM2JkLi5lZjRm OTU0MDk3MjMgMTAwNjQ0Ci0tLSBhL21tL2todWdlcGFnZWQuYworKysgYi9tbS9raHVnZXBhZ2Vk LmMKQEAgLTIyOTUsNiArMjI5NSwxOSBAQCBzdGF0aWMgaW50IGhwYWdlX2NvbGxhcHNlX3NjYW5f ZmlsZShzdHJ1Y3QgbW1fc3RydWN0ICptbSwgdW5zaWduZWQgbG9uZyBhZGRyLAogCQkJY29udGlu dWU7CiAJCX0KIAorCQkvKiBUYWtlIGEgcmVmZXJlbmNlIHRvIHByZXZlbnQgYW55IGNvbmN1cnJl bnQgc3BsaXQgb3IgZnJlZS4gKi8KKwkJaWYgKCFmb2xpb190cnlfZ2V0KGZvbGlvKSkgeworCQkJ eGFzX3Jlc2V0KCZ4YXMpOworCQkJY29udGludWU7CisJCX0KKworCQkvKiBIYXMgdGhlIGZvbGlv IGJlZW4gZnJlZWQgb3Igc3BsaXQ/ICovCisJCWlmICh1bmxpa2VseShmb2xpbyAhPSB4YXNfcmVs b2FkKCZ4YXMpKSkgeworCQkJZm9saW9fcHV0KGZvbGlvKTsKKwkJCXhhc19yZXNldCgmeGFzKTsK KwkJCWNvbnRpbnVlOworCQl9CisKIAkJaWYgKGZvbGlvX29yZGVyKGZvbGlvKSA9PSBIUEFHRV9Q TURfT1JERVIgJiYKIAkJICAgIGZvbGlvLT5pbmRleCA9PSBzdGFydCkgewogCQkJLyogTWF5YmUg UE1ELW1hcHBlZCAqLwpAQCAtMjMwNSwyMyArMjMxOCwyNyBAQCBzdGF0aWMgaW50IGhwYWdlX2Nv bGxhcHNlX3NjYW5fZmlsZShzdHJ1Y3QgbW1fc3RydWN0ICptbSwgdW5zaWduZWQgbG9uZyBhZGRy LAogCQkJICogaXQncyBzYWZlIHRvIHNraXAgTFJVIGFuZCByZWZjb3VudCBjaGVja3MgYmVmb3Jl CiAJCQkgKiByZXR1cm5pbmcuCiAJCQkgKi8KKwkJCWZvbGlvX3B1dChmb2xpbyk7CiAJCQlicmVh azsKIAkJfQogCiAJCW5vZGUgPSBmb2xpb19uaWQoZm9saW8pOwogCQlpZiAoaHBhZ2VfY29sbGFw c2Vfc2Nhbl9hYm9ydChub2RlLCBjYykpIHsKIAkJCXJlc3VsdCA9IFNDQU5fU0NBTl9BQk9SVDsK KwkJCWZvbGlvX3B1dChmb2xpbyk7CiAJCQlicmVhazsKIAkJfQogCQljYy0+bm9kZV9sb2FkW25v ZGVdKys7CiAKIAkJaWYgKCFmb2xpb190ZXN0X2xydShmb2xpbykpIHsKIAkJCXJlc3VsdCA9IFND QU5fUEFHRV9MUlU7CisJCQlmb2xpb19wdXQoZm9saW8pOwogCQkJYnJlYWs7CiAJCX0KIAogCQlp ZiAoIWlzX3JlZmNvdW50X3N1aXRhYmxlKGZvbGlvKSkgewogCQkJcmVzdWx0ID0gU0NBTl9QQUdF X0NPVU5UOworCQkJZm9saW9fcHV0KGZvbGlvKTsKIAkJCWJyZWFrOwogCQl9CiAKQEAgLTIzMzMs NiArMjM1MCw3IEBAIHN0YXRpYyBpbnQgaHBhZ2VfY29sbGFwc2Vfc2Nhbl9maWxlKHN0cnVjdCBt bV9zdHJ1Y3QgKm1tLCB1bnNpZ25lZCBsb25nIGFkZHIsCiAJCSAqLwogCiAJCXByZXNlbnQgKz0g Zm9saW9fbnJfcGFnZXMoZm9saW8pOworCQlmb2xpb19wdXQoZm9saW8pOwogCiAJCWlmIChuZWVk X3Jlc2NoZWQoKSkgewogCQkJeGFzX3BhdXNlKCZ4YXMpOwotLSAKMi4zNC4xCgo= --------------GXQVz0EIt8DBlOQ1kU3p0V3K--