Warning on isolate tail page in isolate_lru_page

[mawupeng <mawupeng1@huawei.com>]

Hi

Recently I receive a warning in isolate_lru_page() reported by syzkaller.
This warning occurred in linux-v5.10 and can't be be reproduced.

The following two commits are the major changes since v5.10:

Commit ac1e9acc5acf ("mm: rearrange madvise code to allow for reuse") refactor
do_madvise in master and lead to call madvise_vma_behavior() instead of
madvise_vma(). For page out these is no difference because both all them will
call madvise_pageout() int the end.

Commit a72afd873089 ("tlb: mmu_gather: Remove start/end arguments from tlb_gather_mmu()")
remove start/end arguments from tlb_gather_mmu in madvise_pageout() since they
are no longer needed.

Warn msg "trying to isolate tail page" will be reported in isolate_lru_page()
if this page is a tail page.

However, if this page is a tail page, it will be split in
madvise_cold_or_pageout_pte_range(). Read lock mmap_read_lock(mm) is hold since
do_madvise() so no one can modify this. So the only reason I can image is
that something is wrong in split_huge_page().

do_madvise
     mmap_read_lock(mm);
         madvise_pageout
             madvise_cold_or_pageout_pte_range
                 split_huge_page(page) <-- split this huge page
                 isolate_lru_page(page)
                     WARN_RATELIMIT(PageTail(page), "trying to isolate tail page");

The warning log is shown below:
==============================================================

WARNING: CPU: 1 PID: 26735 at mm/vmscan.c:1968 isolate_lru_page+0x44d/0x460 mm/vmscan.c:1968
Modules linked in:
RAX: 06bc73006006b800 RBX: 0000000000000001 RCX: 0000000009400000
RDX: ffffc90016103000 RSI: 0000000000000344 RDI: 0000000000000345
RBP: 0000000000000001 R08: ffffffff8a58bab9 R09: ffffed100c4c4f23
R10: ffffed100c4c4f23 R11: 1ffff1100c4c4f22 R12: ffffea0001d59a00
R13: ffffea0001d59bc0 R14: ffffea0001d59bc8 R15: 0000000020ffb000
FS:  00007f00b4284700(0000) GS:ffff88811b280000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000007541ff CR3: 0000000033dec004 CR4: 00000000003706e0
Call Trace:
  madvise_cold_or_pageout_pte_range+0x511/0x6d0 mm/madvise.c:460
  walk_pmd_range mm/pagewalk.c:89 [inline]
  walk_pud_range mm/pagewalk.c:160 [inline]
  walk_p4d_range+0x7f3/0xdb0 mm/pagewalk.c:193
  walk_pgd_range+0x2d3/0x360 mm/pagewalk.c:229
  __walk_page_range+0xda/0x360 mm/pagewalk.c:331
  walk_page_range+0x166/0x380 mm/pagewalk.c:427
  madvise_vma mm/madvise.c:520 [inline]
  do_madvise+0x159d/0x1810 mm/madvise.c:1137
  __do_sys_madvise mm/madvise.c:1163 [inline]
  __se_sys_madvise mm/madvise.c:1161 [inline]
  __x64_sys_madvise+0x5d/0x70 mm/madvise.c:1161
  do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

I have no idea how to fix this warning, so is there anything else need to
analysis that I haven't considered?

Thanks.