Re: [PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jun 20, 2026 at 04:48:57PM -0700, Suren Baghdasaryan wrote:
> Just checking in on the followup plans. IIUC the RFC mentioned will
> try to implement the solution we discussed at LSFMM: splitting
> VM_FAULT_RETRY into two flags - one for retrying under per-VMA locks
> and another one to fallback to mmap_lock.

I continue to hate this idea.  I don't believe that those who were
pushing for it have ever tried to understand the whole fault path.
It's utterly byzantine.

I defy anyone to make sense of this:

        /*
         * NOTE! This will make us return with VM_FAULT_RETRY, but with
         * the fault lock still held. That's how FAULT_FLAG_RETRY_NOWAIT
         * is supposed to work. We have way too many special cases..
         */
        if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
                return 0;

        *fpin = maybe_unlock_mmap_for_io(vmf, *fpin);
        if (vmf->flags & FAULT_FLAG_KILLABLE) {
                if (__folio_lock_killable(folio)) {
                        /*
                         * We didn't have the right flags to drop the
                         * fault lock, but all fault_handlers only check
                         * for fatal signals if we return VM_FAULT_RETRY,
                         * so we need to drop the fault lock here and
                         * return 0 if we don't have a fpin.
                         */
                        if (*fpin == NULL)
                                release_fault_lock(vmf);
                        return 0;
                }

Wed need to simplify the fault path, not add additional complexity.
Josef has said he wouldn't've done the lock dropping had we had per-VMA
locks.  We should rip it out.