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Date: Mon, 09 Sep 2013 08:14:46 +0200
From: Krzysztof Kozlowski <k.kozlowski@...sung.com>
To: Bob Liu <bob.liu@...cle.com>
Cc: Seth Jennings <sjenning@...ux.vnet.ibm.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Mel Gorman <mgorman@...e.de>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
Marek Szyprowski <m.szyprowski@...sung.com>,
Kyungmin Park <kyungmin.park@...sung.com>,
Dave Hansen <dave.hansen@...el.com>,
Minchan Kim <minchan@...nel.org>,
Tomasz Stanislawski <t.stanislaws@...sung.com>
Subject: Re: [RFC PATCH 1/4] zbud: use page ref counter for zbud pages
Hi Bob,
On nie, 2013-09-08 at 17:04 +0800, Bob Liu wrote:
> Hi Krzysztof,
>
> On 08/30/2013 04:42 PM, Krzysztof Kozlowski wrote:
> > Use page reference counter for zbud pages. The ref counter replaces
> > zbud_header.under_reclaim flag and ensures that zbud page won't be freed
> > when zbud_free() is called during reclaim. It allows implementation of
> > additional reclaim paths.
> >
> > The page count is incremented when:
> > - a handle is created and passed to zswap (in zbud_alloc()),
> > - user-supplied eviction callback is called (in zbud_reclaim_page()).
> >
> > Signed-off-by: Krzysztof Kozlowski <k.kozlowski@...sung.com>
> > Signed-off-by: Tomasz Stanislawski <t.stanislaws@...sung.com>
> > Reviewed-by: Bob Liu <bob.liu@...cle.com>
>
> AFAIR, the previous version you sent out has a function called
> rebalance_lists() which I think is a good clean up.
> But I didn't see that function any more in this version.
Yes, that function was added because similar code was present in
zbud_free/zbud_alloc/zbud_reclaim_page. I removed it because it turned
out that there is no benefit in generalizing this code. Seth found an
issue in this function (zbud page was re-inserted in to LRU when
zbud_free() was called on one buddy). Fixing this issue added more if-s
and more code thus enlarging the rebalance_lists() function.
Best regards,
Krzysztof
> Thanks,
> -Bob
>
>
> > ---
> > mm/zbud.c | 97 +++++++++++++++++++++++++++++++++----------------------------
> > 1 file changed, 52 insertions(+), 45 deletions(-)
> >
> > diff --git a/mm/zbud.c b/mm/zbud.c
> > index ad1e781..aa9a15c 100644
> > --- a/mm/zbud.c
> > +++ b/mm/zbud.c
> > @@ -109,7 +109,6 @@ struct zbud_header {
> > struct list_head lru;
> > unsigned int first_chunks;
> > unsigned int last_chunks;
> > - bool under_reclaim;
> > };
> >
> > /*****************
> > @@ -138,16 +137,9 @@ static struct zbud_header *init_zbud_page(struct page *page)
> > zhdr->last_chunks = 0;
> > INIT_LIST_HEAD(&zhdr->buddy);
> > INIT_LIST_HEAD(&zhdr->lru);
> > - zhdr->under_reclaim = 0;
> > return zhdr;
> > }
> >
> > -/* Resets the struct page fields and frees the page */
> > -static void free_zbud_page(struct zbud_header *zhdr)
> > -{
> > - __free_page(virt_to_page(zhdr));
> > -}
> > -
> > /*
> > * Encodes the handle of a particular buddy within a zbud page
> > * Pool lock should be held as this function accesses first|last_chunks
> > @@ -188,6 +180,31 @@ static int num_free_chunks(struct zbud_header *zhdr)
> > return NCHUNKS - zhdr->first_chunks - zhdr->last_chunks - 1;
> > }
> >
> > +/*
> > + * Increases ref count for zbud page.
> > + */
> > +static void get_zbud_page(struct zbud_header *zhdr)
> > +{
> > + get_page(virt_to_page(zhdr));
> > +}
> > +
> > +/*
> > + * Decreases ref count for zbud page and frees the page if it reaches 0
> > + * (no external references, e.g. handles).
> > + *
> > + * Returns 1 if page was freed and 0 otherwise.
> > + */
> > +static int put_zbud_page(struct zbud_header *zhdr)
> > +{
> > + struct page *page = virt_to_page(zhdr);
> > + if (put_page_testzero(page)) {
> > + free_hot_cold_page(page, 0);
> > + return 1;
> > + }
> > + return 0;
> > +}
> > +
> > +
> > /*****************
> > * API Functions
> > *****************/
> > @@ -250,7 +267,7 @@ void zbud_destroy_pool(struct zbud_pool *pool)
> > int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> > unsigned long *handle)
> > {
> > - int chunks, i, freechunks;
> > + int chunks, i;
> > struct zbud_header *zhdr = NULL;
> > enum buddy bud;
> > struct page *page;
> > @@ -273,6 +290,7 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> > bud = FIRST;
> > else
> > bud = LAST;
> > + get_zbud_page(zhdr);
> > goto found;
> > }
> > }
> > @@ -284,6 +302,10 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
> > return -ENOMEM;
> > spin_lock(&pool->lock);
> > pool->pages_nr++;
> > + /*
> > + * We will be using zhdr instead of page, so
> > + * don't increase the page count.
> > + */
> > zhdr = init_zbud_page(page);
> > bud = FIRST;
> >
> > @@ -295,7 +317,7 @@ found:
> >
> > if (zhdr->first_chunks == 0 || zhdr->last_chunks == 0) {
> > /* Add to unbuddied list */
> > - freechunks = num_free_chunks(zhdr);
> > + int freechunks = num_free_chunks(zhdr);
> > list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
> > } else {
> > /* Add to buddied list */
> > @@ -326,7 +348,6 @@ found:
> > void zbud_free(struct zbud_pool *pool, unsigned long handle)
> > {
> > struct zbud_header *zhdr;
> > - int freechunks;
> >
> > spin_lock(&pool->lock);
> > zhdr = handle_to_zbud_header(handle);
> > @@ -337,26 +358,19 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
> > else
> > zhdr->first_chunks = 0;
> >
> > - if (zhdr->under_reclaim) {
> > - /* zbud page is under reclaim, reclaim will free */
> > - spin_unlock(&pool->lock);
> > - return;
> > - }
> > -
> > /* Remove from existing buddy list */
> > list_del(&zhdr->buddy);
> >
> > if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> > - /* zbud page is empty, free */
> > list_del(&zhdr->lru);
> > - free_zbud_page(zhdr);
> > pool->pages_nr--;
> > } else {
> > /* Add to unbuddied list */
> > - freechunks = num_free_chunks(zhdr);
> > + int freechunks = num_free_chunks(zhdr);
> > list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
> > }
> >
> > + put_zbud_page(zhdr);
> > spin_unlock(&pool->lock);
> > }
> >
> > @@ -400,7 +414,7 @@ void zbud_free(struct zbud_pool *pool, unsigned long handle)
> > */
> > int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> > {
> > - int i, ret, freechunks;
> > + int i, ret;
> > struct zbud_header *zhdr;
> > unsigned long first_handle = 0, last_handle = 0;
> >
> > @@ -411,11 +425,24 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> > return -EINVAL;
> > }
> > for (i = 0; i < retries; i++) {
> > + if (list_empty(&pool->lru)) {
> > + /*
> > + * LRU was emptied during evict calls in previous
> > + * iteration but put_zbud_page() returned 0 meaning
> > + * that someone still holds the page. This may
> > + * happen when some other mm mechanism increased
> > + * the page count.
> > + * In such case we succedded with reclaim.
> > + */
> > + spin_unlock(&pool->lock);
> > + return 0;
> > + }
> > zhdr = list_tail_entry(&pool->lru, struct zbud_header, lru);
> > + /* Move this last element to beginning of LRU */
> > list_del(&zhdr->lru);
> > - list_del(&zhdr->buddy);
> > + list_add(&zhdr->lru, &pool->lru);
> > /* Protect zbud page against free */
> > - zhdr->under_reclaim = true;
> > + get_zbud_page(zhdr);
> > /*
> > * We need encode the handles before unlocking, since we can
> > * race with free that will set (first|last)_chunks to 0
> > @@ -440,29 +467,9 @@ int zbud_reclaim_page(struct zbud_pool *pool, unsigned int retries)
> > goto next;
> > }
> > next:
> > - spin_lock(&pool->lock);
> > - zhdr->under_reclaim = false;
> > - if (zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
> > - /*
> > - * Both buddies are now free, free the zbud page and
> > - * return success.
> > - */
> > - free_zbud_page(zhdr);
> > - pool->pages_nr--;
> > - spin_unlock(&pool->lock);
> > + if (put_zbud_page(zhdr))
> > return 0;
> > - } else if (zhdr->first_chunks == 0 ||
> > - zhdr->last_chunks == 0) {
> > - /* add to unbuddied list */
> > - freechunks = num_free_chunks(zhdr);
> > - list_add(&zhdr->buddy, &pool->unbuddied[freechunks]);
> > - } else {
> > - /* add to buddied list */
> > - list_add(&zhdr->buddy, &pool->buddied);
> > - }
> > -
> > - /* add to beginning of LRU */
> > - list_add(&zhdr->lru, &pool->lru);
> > + spin_lock(&pool->lock);
> > }
> > spin_unlock(&pool->lock);
> > return -EAGAIN;
> >
--
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