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Date: Fri, 27 May 2016 12:55:41 +0200
From: Vitaly Wool <vitalywool@...il.com>
To: Dan Streetman <ddstreet@...e.org>
Cc: Linux-MM <linux-mm@...ck.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Seth Jennings <sjenning@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v4] z3fold: the 3-fold allocator for compressed pages
Hello Dan,
On Fri, May 20, 2016 at 2:39 PM, Dan Streetman <ddstreet@...e.org> wrote:
<snip>
>> +static int z3fold_compact_page(struct z3fold_header *zhdr)
>> +{
>> + struct page *page = virt_to_page(zhdr);
>> + void *beg = zhdr;
>> +
>> +
>> + if (!test_bit(MIDDLE_CHUNK_MAPPED, &page->private) &&
>> + zhdr->middle_chunks != 0 &&
>> + zhdr->first_chunks == 0 && zhdr->last_chunks == 0) {
>> + memmove(beg + ZHDR_SIZE_ALIGNED,
>> + beg + (zhdr->start_middle << CHUNK_SHIFT),
>> + zhdr->middle_chunks << CHUNK_SHIFT);
>> + zhdr->first_chunks = zhdr->middle_chunks;
>> + zhdr->middle_chunks = 0;
>> + zhdr->start_middle = 0;
>> + zhdr->first_num++;
>> + return 1;
>> + }
>
> what about the case of only first and middle, or only middle and last?
> you can still optimize space in those cases.
That's right, but I ran some performance tests, too, and with those
extra optimizations
z3fold is on par with zsmalloc, while the existing implementation
gives 10-30% better
numbers. OTOH, the gain in space according to my measurements is 5-10% so I am
not that eager to trade performance gain for it. I was thinking of
having theoe extra
optimizations under a module parameter/flag and I'll come up with the
patch soon, but
I'm going to have that disabled by default anyway.
<snip>
>> +static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
>> +{
>> + struct z3fold_header *zhdr;
>> + int freechunks;
>> + struct page *page;
>> + enum buddy bud;
>> +
>> + spin_lock(&pool->lock);
>> + zhdr = handle_to_z3fold_header(handle);
>> + page = virt_to_page(zhdr);
>> +
>> + if (test_bit(PAGE_HEADLESS, &page->private)) {
>> + /* HEADLESS page stored */
>> + bud = HEADLESS;
>> + } else {
>> + bud = (handle - zhdr->first_num) & BUDDY_MASK;
>
> this should use handle_to_buddy()
Thanks, will fix that.
<snip>
>> +static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
>> +{
>> + int i, ret = 0, freechunks;
>> + struct z3fold_header *zhdr;
>> + struct page *page;
>> + unsigned long first_handle = 0, middle_handle = 0, last_handle = 0;
>> +
>> + spin_lock(&pool->lock);
>> + if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
>> + retries == 0) {
>> + spin_unlock(&pool->lock);
>> + return -EINVAL;
>> + }
>> + for (i = 0; i < retries; i++) {
>> + page = list_last_entry(&pool->lru, struct page, lru);
>> + list_del(&page->lru);
>> +
>> + /* Protect z3fold page against free */
>> + set_bit(UNDER_RECLAIM, &page->private);
>> + zhdr = page_address(page);
>> + if (!test_bit(PAGE_HEADLESS, &page->private)) {
>> + list_del(&zhdr->buddy);
>> + /*
>> + * We need encode the handles before unlocking, since
>> + * we can race with free that will set
>> + * (first|last)_chunks to 0
>> + */
>> + first_handle = 0;
>> + last_handle = 0;
>> + middle_handle = 0;
>> + if (zhdr->first_chunks)
>> + first_handle = encode_handle(zhdr, FIRST);
>> + if (zhdr->middle_chunks)
>> + middle_handle = encode_handle(zhdr, MIDDLE);
>> + if (zhdr->last_chunks)
>> + last_handle = encode_handle(zhdr, LAST);
>> + } else {
>> + first_handle = encode_handle(zhdr, HEADLESS);
>> + last_handle = middle_handle = 0;
>> + }
>> +
>> + spin_unlock(&pool->lock);
>> +
>> + /* Issue the eviction callback(s) */
>> + if (middle_handle) {
>> + ret = pool->ops->evict(pool, middle_handle);
>> + if (ret)
>> + goto next;
>> + }
>> + if (first_handle) {
>> + ret = pool->ops->evict(pool, first_handle);
>> + if (ret)
>> + goto next;
>> + }
>> + if (last_handle) {
>> + ret = pool->ops->evict(pool, last_handle);
>> + if (ret)
>> + goto next;
>> + }
>> +next:
>> + spin_lock(&pool->lock);
>> + clear_bit(UNDER_RECLAIM, &page->private);
>> + if ((test_bit(PAGE_HEADLESS, &page->private) && ret == 0) ||
>> + (zhdr->first_chunks == 0 && zhdr->last_chunks == 0 &&
>> + zhdr->middle_chunks == 0)) {
>> + /*
>> + * All buddies are now free, free the z3fold page and
>> + * return success.
>> + */
>> + clear_bit(PAGE_HEADLESS, &page->private);
>> + free_z3fold_page(zhdr);
>> + pool->pages_nr--;
>> + spin_unlock(&pool->lock);
>> + return 0;
>> + } else if (zhdr->first_chunks != 0 &&
>> + zhdr->last_chunks != 0 && zhdr->middle_chunks != 0) {
>
> if this is a HEADLESS page and the reclaim failed, this else-if will
> be checked which isn't good, since the zhdr data doesn't exist for
> headless pages.
Thanks, and that too.
<snip>
>> +static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
>> +{
>> + struct z3fold_header *zhdr;
>> + struct page *page;
>> + enum buddy buddy;
>> +
>> + spin_lock(&pool->lock);
>> + zhdr = handle_to_z3fold_header(handle);
>> + page = virt_to_page(zhdr);
>> +
>> + if (test_bit(PAGE_HEADLESS, &page->private)) {
>> + spin_unlock(&pool->lock);
>> + return;
>> + }
>> +
>> + buddy = handle_to_buddy(handle);
>> + if (buddy == MIDDLE)
>> + clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
>
> maybe it should be compacted here, in case a compaction was missed
> while the middle chunk was mapped?
I was thinking about that but decided not to go for it at least right
now because
I want map() and unmap() functions to be as fast as possible. I am
considering adding a worker thread that would run compaction in an async
mode and here would be one of the places to trigger it then. Does that sound
reasonable to you?
~vitaly
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