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Date: Mon, 21 Sep 2015 12:17:21 -0400
From: Dan Streetman <ddstreet@...e.org>
To: Vitaly Wool <vitalywool@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Minchan Kim <minchan@...nel.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
linux-kernel <linux-kernel@...r.kernel.org>,
Linux-MM <linux-mm@...ck.org>,
Seth Jennings <sjennings@...iantweb.net>
Subject: Re: [PATCH 1/2] zbud: allow PAGE_SIZE allocations
Please make sure to cc Seth also, he's the owner of zbud.
On Wed, Sep 16, 2015 at 7:50 AM, Vitaly Wool <vitalywool@...il.com> wrote:
> For zram to be able to use zbud via the common zpool API,
> allocations of size PAGE_SIZE should be allowed by zpool.
> zbud uses the beginning of an allocated page for its internal
> structure but it is not a problem as long as we keep track of
> such special pages using a newly introduced page flag.
> To be able to keep track of zbud pages in any case, struct page's
> lru pointer will be used for zbud page lists instead of the one
> that used to be part of the aforementioned internal structure.
>
> Signed-off-by: Vitaly Wool <vitalywool@...il.com>
> ---
> include/linux/page-flags.h | 3 ++
> mm/zbud.c | 71 ++++++++++++++++++++++++++++++++++++++--------
> 2 files changed, 62 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 416509e..dd47cf0 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -134,6 +134,9 @@ enum pageflags {
>
> /* SLOB */
> PG_slob_free = PG_private,
> +
> + /* ZBUD */
> + PG_uncompressed = PG_owner_priv_1,
you don't need a new page flag. and there's 0% chance it would be
accepted even if you did.
> };
>
> #ifndef __GENERATING_BOUNDS_H
> diff --git a/mm/zbud.c b/mm/zbud.c
> index fa48bcdf..ee8b5d6 100644
> --- a/mm/zbud.c
> +++ b/mm/zbud.c
> @@ -107,13 +107,11 @@ struct zbud_pool {
> * struct zbud_header - zbud page metadata occupying the first chunk of each
> * zbud page.
> * @buddy: links the zbud page into the unbuddied/buddied lists in the pool
> - * @lru: links the zbud page into the lru list in the pool
> * @first_chunks: the size of the first buddy in chunks, 0 if free
> * @last_chunks: the size of the last buddy in chunks, 0 if free
> */
> struct zbud_header {
> struct list_head buddy;
> - struct list_head lru;
> unsigned int first_chunks;
> unsigned int last_chunks;
> bool under_reclaim;
> @@ -221,6 +219,7 @@ MODULE_ALIAS("zpool-zbud");
> *****************/
> /* Just to make the code easier to read */
> enum buddy {
> + FULL,
> FIRST,
> LAST
> };
> @@ -241,7 +240,7 @@ static struct zbud_header *init_zbud_page(struct page *page)
> zhdr->first_chunks = 0;
> zhdr->last_chunks = 0;
> INIT_LIST_HEAD(&zhdr->buddy);
> - INIT_LIST_HEAD(&zhdr->lru);
> + INIT_LIST_HEAD(&page->lru);
> zhdr->under_reclaim = 0;
> return zhdr;
> }
> @@ -267,11 +266,18 @@ static unsigned long encode_handle(struct zbud_header *zhdr, enum buddy bud)
> * over the zbud header in the first chunk.
> */
> handle = (unsigned long)zhdr;
> - if (bud == FIRST)
> + switch (bud) {
> + case FIRST:
> /* skip over zbud header */
> handle += ZHDR_SIZE_ALIGNED;
> - else /* bud == LAST */
> + break;
> + case LAST:
> handle += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
> + break;
> + case FULL:
> + default:
Hmm, while it should be ok to treat a default (invalid) bud value as a
full page (assuming the caller treats it as such), you should at least
add a pr_warn() or pr_warn_ratelimited(), or maybe a WARN_ON() or
WARN_ON_ONCE(). the default case should never happen, and a warning
should be printed if it does.
> + break;
> + }
> return handle;
> }
>
> @@ -360,6 +366,24 @@ int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
>
> if (!size || (gfp & __GFP_HIGHMEM))
> return -EINVAL;
> +
> + if (size == PAGE_SIZE) {
> + /*
> + * This is a special case. The page will be allocated
> + * and used to store uncompressed data
> + */
well you shouldn't special case only PAGE_SIZE. If zram increases its
max_zpage_size to a value > (PAGE_SIZE - ZHDR_SIZE_ALIGNED -
CHUNK_SIZE) then those compressed pages will fail to store here.
I think it would be better to change the size check to a simple
if (size > PAGE_SIZE)
return -ENOSPC;
then use the existing
> if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
to store the object (which is either a large compressed page, or an
uncompressed page) into the full zbud page. And don't duplicate
everything the function does inside an if (), just update the function
to handle PAGE_SIZE storage.
> + page = alloc_page(gfp);
> + if (!page)
> + return -ENOMEM;
> + spin_lock(&pool->lock);
> + pool->pages_nr++;
> + INIT_LIST_HEAD(&page->lru);
> + page->flags |= PG_uncompressed;
> + list_add(&page->lru, &pool->lru);
> + spin_unlock(&pool->lock);
> + *handle = encode_handle(page_address(page), FULL);
> + return 0;
> + }
> if (size > PAGE_SIZE - ZHDR_SIZE_ALIGNED - CHUNK_SIZE)
> return -ENOSPC;
> chunks = size_to_chunks(size);
> @@ -372,6 +396,7 @@ int zbud_alloc(struct zbud_pool *pool, size_t size, gfp_t gfp,
> zhdr = list_first_entry(&pool->unbuddied[i],
> struct zbud_header, buddy);
> list_del(&zhdr->buddy);
> + page = virt_to_page(zhdr);
> if (zhdr->first_chunks == 0)
> bud = FIRST;
> else
> @@ -406,9 +431,9 @@ found:
> }
>
> /* Add/move zbud page to beginning of LRU */
> - if (!list_empty(&zhdr->lru))
> - list_del(&zhdr->lru);
> - list_add(&zhdr->lru, &pool->lru);
> + if (!list_empty(&page->lru))
> + list_del(&page->lru);
> + list_add(&page->lru, &pool->lru);
>
> *handle = encode_handle(zhdr, bud);
> spin_unlock(&pool->lock);
> @@ -430,9 +455,21 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
> {
> struct zbud_header *zhdr;
> int freechunks;
> + struct page *page;
>
> spin_lock(&pool->lock);
> zhdr = handle_to_zbud_header(handle);
> + page = virt_to_page(zhdr);
> +
> + /* If it was an uncompressed full page, just free it */
> + if (page->flags & PG_uncompressed) {
> + page->flags &= ~PG_uncompressed;
> + list_del(&page->lru);
> + __free_page(page);
> + pool->pages_nr--;
> + spin_unlock(&pool->lock);
> + return;
> + }
don't repeat this function inside an if() block. update the actual
function to handle the new case.
and you don't need a new page flag. you have 3 distinct cases:
switch (handle & ~PAGE_MASK) {
case 0: /* this is a full-sized page */
case ZHDR_SIZE_ALIGNED: /* this is the first buddy */
default: /* this is the last buddy */
}
>
> /* If first buddy, handle will be page aligned */
> if ((handle - ZHDR_SIZE_ALIGNED) & ~PAGE_MASK)
> @@ -451,7 +488,7 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
>
> if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> /* zbud page is empty, free */
> - list_del(&zhdr->lru);
> + list_del(&page->lru);
> free_zbud_page(zhdr);
> pool->pages_nr--;
> } else {
> @@ -505,6 +542,7 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> {
> int i, ret, freechunks;
> struct zbud_header *zhdr;
> + struct page *page;
> unsigned long first_handle = 0, last_handle = 0;
>
> spin_lock(&pool->lock);
> @@ -514,8 +552,17 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> return -EINVAL;
> }
> for (i = 0; i < retries; i++) {
> - zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
> - list_del(&zhdr->lru);
> + page = list_tail_entry(&pool->lru, struct page, lru);
> + zhdr = page_address(page);
> + list_del(&page->lru);
> + /* Uncompressed zbud page? just run eviction and free it */
> + if (page->flags & PG_uncompressed) {
> + page->flags &= ~PG_uncompressed;
> + spin_unlock(&pool->lock);
> + pool->ops->evict(pool, encode_handle(zhdr, FULL));
> + __free_page(page);
> + return 0;
again, don't be redundant. change the function to handle full-sized
pages, don't repeat the function in an if() block for a special case.
> + }
> list_del(&zhdr->buddy);
> /* Protect zbud page against free */
> zhdr->under_reclaim = true;
> @@ -565,7 +612,7 @@ next:
> }
>
> /* add to beginning of LRU */
> - list_add(&zhdr->lru, &pool->lru);
> + list_add(&page->lru, &pool->lru);
> }
> spin_unlock(&pool->lock);
> return -EAGAIN;
> --
> 1.9.1
--
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