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SubjectRe: [PATCH v5 2/3] z3fold: remove redundant locking
On Mon, Oct 17, 2016 at 10:48 PM, Dan Streetman <ddstreet@ieee.org> wrote:
> On Sat, Oct 15, 2016 at 7:59 AM, Vitaly Wool <vitalywool@gmail.com> wrote:
>> The per-pool z3fold spinlock should generally be taken only when
>> a non-atomic pool variable is modified. There's no need to take it
>> to map/unmap an object.
>
> no. it absolutely needs to be taken, because z3fold_compact_page
> could move the middle bud's contents to the first bud, and if the
> middle bud gets mapped while it's being moved really bad things will
> happen.
>
> you can change that to a per-page lock in the z3fold_header, but some
> locking needs to happen between mapping and middle bud moving (and
> handle encoding/decoding and first_num access).

Yep, probably per-page lock is the way to go. I was thinking of making
first_num atomic but that obviously creates more problems than it solves.

~vitaly

>
>
>>
>> Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
>> ---
>> mm/z3fold.c | 17 +++++------------
>> 1 file changed, 5 insertions(+), 12 deletions(-)
>>
>> diff --git a/mm/z3fold.c b/mm/z3fold.c
>> index 5197d7b..10513b5 100644
>> --- a/mm/z3fold.c
>> +++ b/mm/z3fold.c
>> @@ -580,6 +580,7 @@ next:
>> if ((test_bit(PAGE_HEADLESS, &page->private) && ret == 0) ||
>> (zhdr->first_chunks == 0 && zhdr->last_chunks == 0 &&
>> zhdr->middle_chunks == 0)) {
>> + spin_unlock(&pool->lock);
>> /*
>> * All buddies are now free, free the z3fold page and
>> * return success.
>> @@ -587,7 +588,6 @@ next:
>> clear_bit(PAGE_HEADLESS, &page->private);
>> free_z3fold_page(zhdr);
>> atomic64_dec(&pool->pages_nr);
>> - spin_unlock(&pool->lock);
>> return 0;
>> } else if (!test_bit(PAGE_HEADLESS, &page->private)) {
>> if (zhdr->first_chunks != 0 &&
>> @@ -629,7 +629,6 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
>> void *addr;
>> enum buddy buddy;
>>
>> - spin_lock(&pool->lock);
>> zhdr = handle_to_z3fold_header(handle);
>> addr = zhdr;
>> page = virt_to_page(zhdr);
>> @@ -656,7 +655,6 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
>> break;
>> }
>> out:
>> - spin_unlock(&pool->lock);
>> return addr;
>> }
>>
>> @@ -671,19 +669,14 @@ static void z3fold_unmap(struct z3fold_pool *pool, unsigned long handle)
>> 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;
>> + if (!test_bit(PAGE_HEADLESS, &page->private)) {
>> + buddy = handle_to_buddy(handle);
>> + if (buddy == MIDDLE)
>> + clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
>> }
>> -
>> - buddy = handle_to_buddy(handle);
>> - if (buddy == MIDDLE)
>> - clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
>> - spin_unlock(&pool->lock);
>> }
>>
>> /**
>> --
>> 2.4.2

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