Re: [PATCH v2 4/4] mm: try to free swapcache for non-LRU folios

[Barry Song <baohua@kernel.org>]

On Tue, Jun 30, 2026 at 1:48 PM David Hildenbrand (Arm)
<david@kernel.org> wrote:
[...]
>
> -       if (vmf->flags & FAULT_FLAG_WRITE) {
> +       if ((vmf->flags & FAULT_FLAG_WRITE) && !pte_write(pte)) {
>                 ret |= do_wp_page(vmf);
>                 if (ret & VM_FAULT_ERROR)
>                         ret &= VM_FAULT_ERROR;
>

Thanks! I'm working on a v3, but one thing still seems a bit odd to me.

In patch 1, we're removing the local LRU drain from
do_wp_page(). But in patch 4, do_swap_page() calls
do_wp_page() for folios without the "exclusive" hint,
since folios with the hint have already been reused in
do_swap_page().

This can happen when we allocate a new folio whose swap
entry lacks the exclusive flag, even though the folio is
still reusable. For example, in the scenario I mentioned
in patch 1, the swap count is effectively 1, but there is
no exclusive flag:

#include <sys/mman.h>

int main(int argc, char *argv[])
{
    int i;
    #define SIZE 30*1024*1024
    while(1) {
        volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
             MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        volatile int q;

        for (int i = 0; i < SIZE/sizeof(int); i++)
              p[i] =  i;
        madvise((void *)p, SIZE, MADV_PAGEOUT);
        if (!fork())
             _exit(0);
        for (int i = 0; i < SIZE/sizeof(int); i++)
              q = p[i];
        for (int i = 0; i < SIZE/sizeof(int); i++)
              p[i] =  i;
        munmap(p, SIZE);
    }
    return 0;
}

The newly allocated folio will likely still be sitting in this
CPU's local LRU cache. If we drop the drain in patch 1, it seems
there is little point in calling do_wp_page() from
do_swap_page(), since we won't be able to reuse the folio while
it remains in the local LRU cache.

So I'm wondering whether we should keep the drain in
patch 1 by checking for a refcount of 3, or instead do
something like the following in patch 4:


        if ((vmf->flags & FAULT_FLAG_WRITE) && !pte_write(pte)) {
                 lru_add_drain();
                 ret |= do_wp_page(vmf);
                 if (ret & VM_FAULT_ERROR)
                         ret &= VM_FAULT_ERROR;
       }

Otherwise, the combination of patches 1 and 4 seems somewhat
inconsistent. It feels like a strange mix of sweet, spicy,
hot, and cold—kind of contradictory :-)

Best Regards
Barry