CVE-2026-23375: mm: thp: deny THP for files on anonymous inodes

In the Linux kernel, the following vulnerability has been resolved:

mm: thp: deny THP for files on anonymous inodes

filethpenabled() incorrectly allows THP for files on anonymous inodes (e.g. guestmemfd and secretmem). These files are created via allocfilepseudo(), which does not call getwriteaccess() and leaves inode->iwritecount at 0. Combined with SISREG(inode->imode) being true, they appear as read-only regular files when CONFIGREADONLYTHPFORFS is enabled, making them eligible for THP collapse.

Anonymous inodes can never pass the inodeisopenforwrite() check since their iwritecount is never incremented through the normal VFS open path. The right thing to do is to exclude them from THP eligibility altogether, since CONFIGREADONLYTHPFORFS was designed for real filesystem files (e.g. shared libraries), not for pseudo-filesystem inodes.

For guestmemfd, this allows khugepaged and MADVCOLLAPSE to create large folios in the page cache via the collapse path, but the guestmemfd fault handler does not support large folios. This triggers WARNONONCE(foliotestlarge(folio)) in kvmgmemfaultusermapping().

For secretmem, collapsefile() tries to copy page contents through the direct map, but secretmem pages are removed from the direct map. This can result in a kernel crash:

BUG: unable to handle page fault for address: ffff88810284d000 RIP: 0010:memcpyorig+0x16/0x130 Call Trace: collapsefile hpagecollapsescanfile madvisecollapse

Secretmem is not affected by the crash on upstream as the memory failure recovery handles the failed copy gracefully, but it still triggers confusing false memory failure reports:

Memory failure: 0x106d96f: recovery action for clean unevictable LRU page: Recovered

Check ISANONFILE(inode) in filethpenabled() to deny THP for all anonymous inode files.