On Mon, 8 Aug 2022, Matthew Wilcox (Oracle) wrote:
> The 'swapcache' variable is used to track whether the page is from the
> swapcache or not. It can do this equally well by being the folio of
> the page rather than the page itself, and this saves a number of calls
> to compound_head().
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> mm/memory.c | 32 ++++++++++++++++----------------
> 1 file changed, 16 insertions(+), 16 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index f172b148e29b..471102f0cbf2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3718,8 +3718,8 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> vm_fault_t do_swap_page(struct vm_fault *vmf)
> {
> struct vm_area_struct *vma = vmf->vma;
> - struct folio *folio;
> - struct page *page = NULL, *swapcache;
> + struct folio *swapcache, *folio = NULL;
> + struct page *page;
> struct swap_info_struct *si = NULL;
> rmap_t rmap_flags = RMAP_NONE;
> bool exclusive = false;
> @@ -3762,11 +3762,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> goto out;
>
> page = lookup_swap_cache(entry, vma, vmf->address);
> - swapcache = page;
> if (page)
> folio = page_folio(page);
> + swapcache = folio;
>
> - if (!page) {
> + if (!folio) {
> if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> __swap_count(entry) == 1) {
> /* skip swapcache */
> @@ -3799,12 +3799,12 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> } else {
> page = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE,
> vmf);
> - swapcache = page;
> if (page)
> folio = page_folio(page);
> + swapcache = folio;
> }
>
> - if (!page) {
> + if (!folio) {
> /*
> * Back out if somebody else faulted in this pte
> * while we released the pte lock.
> @@ -3856,10 +3856,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> page = ksm_might_need_to_copy(page, vma, vmf->address);
> if (unlikely(!page)) {
> ret = VM_FAULT_OOM;
> - page = swapcache;
> goto out_page;
> }
> folio = page_folio(page);
> + swapcache = folio;
I couldn't get further than one iteration into my swapping loads:
processes hung waiting for the folio lock.
Delete that "swapcache = folio;" line: here is (one place) where
swapcache and folio may diverge, and shall need to be unlocked
and put separately. All working okay since I deleted that.
Hugh
>
> /*
> * If we want to map a page that's in the swapcache writable, we
> @@ -3867,7 +3867,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> * owner. Try removing the extra reference from the local LRU
> * pagevecs if required.
> */
> - if ((vmf->flags & FAULT_FLAG_WRITE) && page == swapcache &&
> + if ((vmf->flags & FAULT_FLAG_WRITE) && folio == swapcache &&
> !folio_test_ksm(folio) && !folio_test_lru(folio))
> lru_add_drain();
> }
> @@ -3908,7 +3908,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> * without __HAVE_ARCH_PTE_SWP_EXCLUSIVE.
> */
> exclusive = pte_swp_exclusive(vmf->orig_pte);
> - if (page != swapcache) {
> + if (folio != swapcache) {
> /*
> * We have a fresh page that is not exposed to the
> * swapcache -> certainly exclusive.
> @@ -3976,7 +3976,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> vmf->orig_pte = pte;
>
> /* ksm created a completely new copy */
> - if (unlikely(page != swapcache && swapcache)) {
> + if (unlikely(folio != swapcache && swapcache)) {
> page_add_new_anon_rmap(page, vma, vmf->address);
> folio_add_lru_vma(folio, vma);
> } else {
> @@ -3989,7 +3989,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
>
> folio_unlock(folio);
> - if (page != swapcache && swapcache) {
> + if (folio != swapcache && swapcache) {
> /*
> * Hold the lock to avoid the swap entry to be reused
> * until we take the PT lock for the pte_same() check
> @@ -3998,8 +3998,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> * so that the swap count won't change under a
> * parallel locked swapcache.
> */
> - unlock_page(swapcache);
> - put_page(swapcache);
> + folio_unlock(swapcache);
> + folio_put(swapcache);
> }
>
> if (vmf->flags & FAULT_FLAG_WRITE) {
> @@ -4023,9 +4023,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> folio_unlock(folio);
> out_release:
> folio_put(folio);
> - if (page != swapcache && swapcache) {
> - unlock_page(swapcache);
> - put_page(swapcache);
> + if (folio != swapcache && swapcache) {
> + folio_unlock(swapcache);
> + folio_put(swapcache);
> }
> if (si)
> put_swap_device(si);
> --
> 2.35.1
