| Message ID | 20200309090940.34130-1-richard.weiyang@linux.alibaba.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm/swap_slots.c: don't reset the cache slot after use | expand |
On Mon, 9 Mar 2020 17:09:40 +0800 Wei Yang <richard.weiyang@linux.alibaba.com> wrote: > Currently we would clear the cache slot if it is used. While this is not > necessary, since this entry would not be used until refilled. > > Leave it untouched and assigned the value directly to entry which makes > the code little more neat. > > Also this patch merges the else and if, since this is the only case we > refill and repeat swap cache. cc Tim, who can hopefully remember how this code works ;) > --- a/mm/swap_slots.c > +++ b/mm/swap_slots.c > @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry) > > swp_entry_t get_swap_page(struct page *page) > { > - swp_entry_t entry, *pentry; > + swp_entry_t entry; > struct swap_slots_cache *cache; > > entry.val = 0; > @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page) > if (cache->slots) { > repeat: > if (cache->nr) { > - pentry = &cache->slots[cache->cur++]; > - entry = *pentry; > - pentry->val = 0; > + entry = cache->slots[cache->cur++]; > cache->nr--; > - } else { > - if (refill_swap_slots_cache(cache)) > - goto repeat; > + } else if (refill_swap_slots_cache(cache)) { > + goto repeat; > } > } > mutex_unlock(&cache->alloc_lock); > -- > 2.20.1 (Apple Git-117)
On 3/9/20 5:48 PM, Andrew Morton wrote: > On Mon, 9 Mar 2020 17:09:40 +0800 Wei Yang <richard.weiyang@linux.alibaba.com> wrote: > >> Currently we would clear the cache slot if it is used. While this is not >> necessary, since this entry would not be used until refilled. >> >> Leave it untouched and assigned the value directly to entry which makes >> the code little more neat. >> >> Also this patch merges the else and if, since this is the only case we >> refill and repeat swap cache. > > cc Tim, who can hopefully remember how this code works ;) > >> --- a/mm/swap_slots.c >> +++ b/mm/swap_slots.c >> @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry) >> >> swp_entry_t get_swap_page(struct page *page) >> { >> - swp_entry_t entry, *pentry; >> + swp_entry_t entry; >> struct swap_slots_cache *cache; >> >> entry.val = 0; >> @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page) >> if (cache->slots) { >> repeat: >> if (cache->nr) { >> - pentry = &cache->slots[cache->cur++]; >> - entry = *pentry; >> - pentry->val = 0; The cache entry was cleared after assignment for defensive programming, So there's little chance I will be using a slot that has been assigned to someone else. When I wrote swap_slots.c, this code was new and I want to make sure that if something went wrong, and I assigned a swap slot that I shouldn't, I will be able to detect quickly as I will only be stepping on entry 0. Otherwise such bug will be harder to detect as we will have two users of some random swap slot stepping on each other. I'm okay if we want to get rid of this logic, now that the code has been working correctly long enough. But I think is good hygiene to clear the cached entry after it has been assigned. >> + entry = cache->slots[cache->cur++]; >> cache->nr--; >> - } else { >> - if (refill_swap_slots_cache(cache)) >> - goto repeat; >> + } else if (refill_swap_slots_cache(cache)) { This change looks fine. >> + goto repeat; >> } Tim
On Tue, Mar 10, 2020 at 11:13:13AM -0700, Tim Chen wrote: >On 3/9/20 5:48 PM, Andrew Morton wrote: >> On Mon, 9 Mar 2020 17:09:40 +0800 Wei Yang <richard.weiyang@linux.alibaba.com> wrote: >> >>> Currently we would clear the cache slot if it is used. While this is not >>> necessary, since this entry would not be used until refilled. >>> >>> Leave it untouched and assigned the value directly to entry which makes >>> the code little more neat. >>> >>> Also this patch merges the else and if, since this is the only case we >>> refill and repeat swap cache. >> >> cc Tim, who can hopefully remember how this code works ;) >> >>> --- a/mm/swap_slots.c >>> +++ b/mm/swap_slots.c >>> @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry) >>> >>> swp_entry_t get_swap_page(struct page *page) >>> { >>> - swp_entry_t entry, *pentry; >>> + swp_entry_t entry; >>> struct swap_slots_cache *cache; >>> >>> entry.val = 0; >>> @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page) >>> if (cache->slots) { >>> repeat: >>> if (cache->nr) { >>> - pentry = &cache->slots[cache->cur++]; >>> - entry = *pentry; >>> - pentry->val = 0; > >The cache entry was cleared after assignment for defensive programming, So there's >little chance I will be using a slot that has been assigned to someone else. >When I wrote swap_slots.c, this code was new and I want to make sure >that if something went wrong, and I assigned a swap slot that I shouldn't, >I will be able to detect quickly as I will only be stepping on entry 0. > >Otherwise such bug will be harder to detect as we will have two users of some random >swap slot stepping on each other. > >I'm okay if we want to get rid of this logic, now that the code has been >working correctly long enough. But I think is good hygiene to clear the >cached entry after it has been assigned. > This is fine to keep the logic, while I am wondering whether we need to do this through pointer. cache->slots[] contain the value, we can get and reset without pointer. The following code looks more obvious about the logic. entry = cache->slots[cache->cur]; cache->slots[cache->cur++].val = 0; >>> + entry = cache->slots[cache->cur++]; >>> cache->nr--; >>> - } else { >>> - if (refill_swap_slots_cache(cache)) >>> - goto repeat; >>> + } else if (refill_swap_slots_cache(cache)) { > >This change looks fine. >>> + goto repeat; >>> } > >Tim
On 3/10/20 3:20 PM, Wei Yang wrote: > On Tue, Mar 10, 2020 at 11:13:13AM -0700, Tim Chen wrote: >> On 3/9/20 5:48 PM, Andrew Morton wrote: >>> On Mon, 9 Mar 2020 17:09:40 +0800 Wei Yang <richard.weiyang@linux.alibaba.com> wrote: >>> >>>> Currently we would clear the cache slot if it is used. While this is not >>>> necessary, since this entry would not be used until refilled. >>>> >>>> Leave it untouched and assigned the value directly to entry which makes >>>> the code little more neat. >>>> >>>> Also this patch merges the else and if, since this is the only case we >>>> refill and repeat swap cache. >>> >>> cc Tim, who can hopefully remember how this code works ;) >>> >>>> --- a/mm/swap_slots.c >>>> +++ b/mm/swap_slots.c >>>> @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry) >>>> >>>> swp_entry_t get_swap_page(struct page *page) >>>> { >>>> - swp_entry_t entry, *pentry; >>>> + swp_entry_t entry; >>>> struct swap_slots_cache *cache; >>>> >>>> entry.val = 0; >>>> @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page) >>>> if (cache->slots) { >>>> repeat: >>>> if (cache->nr) { >>>> - pentry = &cache->slots[cache->cur++]; >>>> - entry = *pentry; >>>> - pentry->val = 0; >> >> The cache entry was cleared after assignment for defensive programming, So there's >> little chance I will be using a slot that has been assigned to someone else. >> When I wrote swap_slots.c, this code was new and I want to make sure >> that if something went wrong, and I assigned a swap slot that I shouldn't, >> I will be able to detect quickly as I will only be stepping on entry 0. >> >> Otherwise such bug will be harder to detect as we will have two users of some random >> swap slot stepping on each other. >> >> I'm okay if we want to get rid of this logic, now that the code has been >> working correctly long enough. But I think is good hygiene to clear the >> cached entry after it has been assigned. >> > > This is fine to keep the logic, while I am wondering whether we need to do > this through pointer. cache->slots[] contain the value, we can get and reset > without pointer. > > The following code looks more obvious about the logic. > > entry = cache->slots[cache->cur]; > cache->slots[cache->cur++].val = 0; Yes, this looks pretty good. Thanks. Tim
On Tue, Mar 10, 2020 at 04:03:07PM -0700, Tim Chen wrote: >On 3/10/20 3:20 PM, Wei Yang wrote: >> On Tue, Mar 10, 2020 at 11:13:13AM -0700, Tim Chen wrote: >>> On 3/9/20 5:48 PM, Andrew Morton wrote: >>>> On Mon, 9 Mar 2020 17:09:40 +0800 Wei Yang <richard.weiyang@linux.alibaba.com> wrote: >>>> >>>>> Currently we would clear the cache slot if it is used. While this is not >>>>> necessary, since this entry would not be used until refilled. >>>>> >>>>> Leave it untouched and assigned the value directly to entry which makes >>>>> the code little more neat. >>>>> >>>>> Also this patch merges the else and if, since this is the only case we >>>>> refill and repeat swap cache. >>>> >>>> cc Tim, who can hopefully remember how this code works ;) >>>> >>>>> --- a/mm/swap_slots.c >>>>> +++ b/mm/swap_slots.c >>>>> @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry) >>>>> >>>>> swp_entry_t get_swap_page(struct page *page) >>>>> { >>>>> - swp_entry_t entry, *pentry; >>>>> + swp_entry_t entry; >>>>> struct swap_slots_cache *cache; >>>>> >>>>> entry.val = 0; >>>>> @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page) >>>>> if (cache->slots) { >>>>> repeat: >>>>> if (cache->nr) { >>>>> - pentry = &cache->slots[cache->cur++]; >>>>> - entry = *pentry; >>>>> - pentry->val = 0; >>> >>> The cache entry was cleared after assignment for defensive programming, So there's >>> little chance I will be using a slot that has been assigned to someone else. >>> When I wrote swap_slots.c, this code was new and I want to make sure >>> that if something went wrong, and I assigned a swap slot that I shouldn't, >>> I will be able to detect quickly as I will only be stepping on entry 0. >>> >>> Otherwise such bug will be harder to detect as we will have two users of some random >>> swap slot stepping on each other. >>> >>> I'm okay if we want to get rid of this logic, now that the code has been >>> working correctly long enough. But I think is good hygiene to clear the >>> cached entry after it has been assigned. >>> >> >> This is fine to keep the logic, while I am wondering whether we need to do >> this through pointer. cache->slots[] contain the value, we can get and reset >> without pointer. >> >> The following code looks more obvious about the logic. >> >> entry = cache->slots[cache->cur]; >> cache->slots[cache->cur++].val = 0; > >Yes, this looks pretty good. Thanks, I would rephrase v2. > >Thanks. > >Tim
diff --git a/mm/swap_slots.c b/mm/swap_slots.c index 63a7b4563a57..ff695df3db26 100644 --- a/mm/swap_slots.c +++ b/mm/swap_slots.c @@ -309,7 +309,7 @@ int free_swap_slot(swp_entry_t entry) swp_entry_t get_swap_page(struct page *page) { - swp_entry_t entry, *pentry; + swp_entry_t entry; struct swap_slots_cache *cache; entry.val = 0; @@ -336,13 +336,10 @@ swp_entry_t get_swap_page(struct page *page) if (cache->slots) { repeat: if (cache->nr) { - pentry = &cache->slots[cache->cur++]; - entry = *pentry; - pentry->val = 0; + entry = cache->slots[cache->cur++]; cache->nr--; - } else { - if (refill_swap_slots_cache(cache)) - goto repeat; + } else if (refill_swap_slots_cache(cache)) { + goto repeat; } } mutex_unlock(&cache->alloc_lock);
Currently we would clear the cache slot if it is used. While this is not necessary, since this entry would not be used until refilled. Leave it untouched and assigned the value directly to entry which makes the code little more neat. Also this patch merges the else and if, since this is the only case we refill and repeat swap cache. Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com> --- mm/swap_slots.c | 11 ++++------- 1 file changed, 4 insertions(+), 7 deletions(-)