Re: [PATCH 05/18] mm: memcontrol: convert page cache to a new mem_cgroup_charge() API

Joonsoo Kim
2020-04-22T06:40:50Z

On Mon, Apr 20, 2020 at 06:11:13PM -0400, Johannes Weiner wrote:
> The try/commit/cancel protocol that memcg uses dates back to when
> pages used to be uncharged upon removal from the page cache, and thus
> couldn't be committed before the insertion had succeeded. Nowadays,
> pages are uncharged when they are physically freed; it doesn't matter
> whether the insertion was successful or not. For the page cache, the
> transaction dance has become unnecessary.
>
> Introduce a mem_cgroup_charge() function that simply charges a newly
> allocated page to a cgroup and sets up page->mem_cgroup in one single
> step. If the insertion fails, the caller doesn't have to do anything
> but free/put the page.
>
> Then switch the page cache over to this new API.
>
> Subsequent patches will also convert anon pages, but it needs a bit
> more prep work. Right now, memcg depends on page->mapping being
> already set up at the time of charging, so that it can maintain its
> own MEMCG_CACHE and MEMCG_RSS counters. For anon, page->mapping is set
> under the same pte lock under which the page is publishd, so a single
> charge point that can block doesn't work there just yet.
>
> The following prep patches will replace the private memcg counters
> with the generic vmstat counters, thus removing the page->mapping
> dependency, then complete the transition to the new single-point
> charge API and delete the old transactional scheme.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
> include/linux/memcontrol.h | 10 ++++
> mm/filemap.c | 24 ++++------
> mm/memcontrol.c | 27 +++++++++++
> mm/shmem.c | 97 +++++++++++++++++---------------------
> 4 files changed, 89 insertions(+), 69 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index c7875a48c8c1..5e8b0e38f145 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -367,6 +367,10 @@ int mem_cgroup_try_charge_delay(struct page *page, struct mm_struct *mm,
> void mem_cgroup_commit_charge(struct page *page, struct mem_cgroup *memcg,
> bool lrucare);
> void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg);
> +
> +int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask,
> + bool lrucare);
> +
> void mem_cgroup_uncharge(struct page *page);
> void mem_cgroup_uncharge_list(struct list_head *page_list);
>
> @@ -872,6 +876,12 @@ static inline void mem_cgroup_cancel_charge(struct page *page,
> {
> }
>
> +static inline int mem_cgroup_charge(struct page *page, struct mm_struct *mm,
> + gfp_t gfp_mask, bool lrucare)
> +{
> + return 0;
> +}
> +
> static inline void mem_cgroup_uncharge(struct page *page)
> {
> }
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 5b31af9d5b1b..5bdbda965177 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -832,7 +832,6 @@ static int __add_to_page_cache_locked(struct page *page,
> {
> XA_STATE(xas, &mapping->i_pages, offset);
> int huge = PageHuge(page);
> - struct mem_cgroup *memcg;
> int error;
> void *old;
>
> @@ -840,17 +839,16 @@ static int __add_to_page_cache_locked(struct page *page,
> VM_BUG_ON_PAGE(PageSwapBacked(page), page);
> mapping_set_update(&xas, mapping);
>
> - if (!huge) {
> - error = mem_cgroup_try_charge(page, current->mm,
> - gfp_mask, &memcg);
> - if (error)
> - return error;
> - }
> -
> get_page(page);
> page->mapping = mapping;
> page->index = offset;
>
> + if (!huge) {
> + error = mem_cgroup_charge(page, current->mm, gfp_mask, false);
> + if (error)
> + goto error;
> + }
> +
> do {
> xas_lock_irq(&xas);
> old = xas_load(&xas);
> @@ -874,20 +872,18 @@ static int __add_to_page_cache_locked(struct page *page,
> xas_unlock_irq(&xas);
> } while (xas_nomem(&xas, gfp_mask & GFP_RECLAIM_MASK));
>
> - if (xas_error(&xas))
> + if (xas_error(&xas)) {
> + error = xas_error(&xas);
> goto error;
> + }
>
> - if (!huge)
> - mem_cgroup_commit_charge(page, memcg, false);
> trace_mm_filemap_add_to_page_cache(page);
> return 0;
> error:
> page->mapping = NULL;
> /* Leave page->index set: truncation relies upon it */
> - if (!huge)
> - mem_cgroup_cancel_charge(page, memcg);
> put_page(page);
> - return xas_error(&xas);
> + return error;
> }
> ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 711d6dd5cbb1..b38c0a672d26 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6577,6 +6577,33 @@ void mem_cgroup_cancel_charge(struct page *page, struct mem_cgroup *memcg)
> cancel_charge(memcg, nr_pages);
> }
>
> +/**
> + * mem_cgroup_charge - charge a newly allocated page to a cgroup
> + * @page: page to charge
> + * @mm: mm context of the victim
> + * @gfp_mask: reclaim mode
> + * @lrucare: page might be on the LRU already
> + *
> + * Try to charge @page to the memcg that @mm belongs to, reclaiming
> + * pages according to @gfp_mask if necessary.
> + *
> + * Returns 0 on success. Otherwise, an error code is returned.
> + */
> +int mem_cgroup_charge(struct page *page, struct mm_struct *mm, gfp_t gfp_mask,
> + bool lrucare)
> +{
> + struct mem_cgroup *memcg;
> + int ret;
> +
> + VM_BUG_ON_PAGE(!page->mapping, page);
> +
> + ret = mem_cgroup_try_charge(page, mm, gfp_mask, &memcg);
> + if (ret)
> + return ret;
> + mem_cgroup_commit_charge(page, memcg, lrucare);
> + return 0;
> +}
> +
> struct uncharge_gather {
> struct mem_cgroup *memcg;
> unsigned long pgpgout;
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 52c66801321e..2384f6c7ef71 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -605,11 +605,13 @@ static inline bool is_huge_enabled(struct shmem_sb_info *sbinfo)
> */
> static int shmem_add_to_page_cache(struct page *page,
> struct address_space *mapping,
> - pgoff_t index, void *expected, gfp_t gfp)
> + pgoff_t index, void *expected, gfp_t gfp,
> + struct mm_struct *charge_mm)
> {
> XA_STATE_ORDER(xas, &mapping->i_pages, index, compound_order(page));
> unsigned long i = 0;
> unsigned long nr = compound_nr(page);
> + int error;
>
> VM_BUG_ON_PAGE(PageTail(page), page);
> VM_BUG_ON_PAGE(index != round_down(index, nr), page);
> @@ -621,6 +623,16 @@ static int shmem_add_to_page_cache(struct page *page,
> page->mapping = mapping;
> page->index = index;
>
> + error = mem_cgroup_charge(page, charge_mm, gfp, PageSwapCache(page));
> + if (error) {
> + if (!PageSwapCache(page) && PageTransHuge(page)) {
> + count_vm_event(THP_FILE_FALLBACK);
> + count_vm_event(THP_FILE_FALLBACK_CHARGE);
> + }
> + goto error;
> + }
> + cgroup_throttle_swaprate(page, gfp);
> +
> do {
> void *entry;
> xas_lock_irq(&xas);
> @@ -648,12 +660,15 @@ static int shmem_add_to_page_cache(struct page *page,
> } while (xas_nomem(&xas, gfp));
>
> if (xas_error(&xas)) {
> - page->mapping = NULL;
> - page_ref_sub(page, nr);
> - return xas_error(&xas);
> + error = xas_error(&xas);
> + goto error;
> }
>
> return 0;
> +error:
> + page->mapping = NULL;
> + page_ref_sub(page, nr);
> + return error;
> }
>
> /*
> @@ -1619,7 +1634,6 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
> struct address_space *mapping = inode->i_mapping;
> struct shmem_inode_info *info = SHMEM_I(inode);
> struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
> - struct mem_cgroup *memcg;
> struct page *page;
> swp_entry_t swap;
> int error;
> @@ -1664,29 +1678,22 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
> goto failed;
> }
>
> - error = mem_cgroup_try_charge_delay(page, charge_mm, gfp, &memcg);
> - if (!error) {
> - error = shmem_add_to_page_cache(page, mapping, index,
> - swp_to_radix_entry(swap), gfp);
> - /*
> - * We already confirmed swap under page lock, and make
> - * no memory allocation here, so usually no possibility
> - * of error; but free_swap_and_cache() only trylocks a
> - * page, so it is just possible that the entry has been
> - * truncated or holepunched since swap was confirmed.
> - * shmem_undo_range() will have done some of the
> - * unaccounting, now delete_from_swap_cache() will do
> - * the rest.
> - */
> - if (error) {
> - mem_cgroup_cancel_charge(page, memcg);
> - delete_from_swap_cache(page);
> - }
> - }
> - if (error)
> + error = shmem_add_to_page_cache(page, mapping, index,
> + swp_to_radix_entry(swap), gfp,
> + charge_mm);
> + /*
> + * We already confirmed swap under page lock, and make no
> + * memory allocation here, so usually no possibility of error;
> + * but free_swap_and_cache() only trylocks a page, so it is
> + * just possible that the entry has been truncated or
> + * holepunched since swap was confirmed. shmem_undo_range()
> + * will have done some of the unaccounting, now
> + * delete_from_swap_cache() will do the rest.
> + */
> + if (error) {
> + delete_from_swap_cache(page);
> goto failed;

-EEXIST (from swap cache) and -ENOMEM (from memcg) should be handled
differently. delete_from_swap_cache() is for -EEXIST case.

Thanks.