On Wed, Apr 10, 2013 at 9:19 AM, Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx> wrote: > On Wed, Apr 10, 2013 at 2:28 AM, Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx> wrote: >> In pageset_set_batch() and setup_pagelist_highmark(), ensure that batch >> is always set to a safe value (1) prior to updating high, and ensure >> that high is fully updated before setting the real value of batch. >> >> Suggested by Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx> in this thread: >> >> https://lkml.org/lkml/2013/4/9/23 >> >> Also reproduces his proposed comment. >> >> Signed-off-by: Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx> >> --- >> mm/page_alloc.c | 19 +++++++++++++++++++ >> 1 file changed, 19 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index d259599..a07bd4c 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4007,11 +4007,26 @@ static int __meminit zone_batchsize(struct zone *zone) >> #endif >> } >> >> +static void pageset_update_prep(struct per_cpu_pages *pcp) >> +{ >> + /* >> + * We're about to mess with PCP in an non atomic fashion. Put an >> + * intermediate safe value of batch and make sure it is visible before >> + * any other change >> + */ >> + pcp->batch = 1; >> + smp_wmb(); >> +} >> + >> /* a companion to setup_pagelist_highmark() */ >> static void pageset_set_batch(struct per_cpu_pageset *p, unsigned long batch) >> { >> struct per_cpu_pages *pcp = &p->pcp; >> + pageset_update_prep(pcp); >> + >> pcp->high = 6 * batch; >> + smp_wmb(); >> + >> pcp->batch = max(1UL, 1 * batch); >> } >> >> @@ -4039,7 +4054,11 @@ static void setup_pagelist_highmark(struct per_cpu_pageset *p, >> struct per_cpu_pages *pcp; >> >> pcp = &p->pcp; >> + pageset_update_prep(pcp); >> + >> pcp->high = high; >> + smp_wmb(); >> + >> pcp->batch = max(1UL, high/4); >> if ((high/4) > (PAGE_SHIFT * 8)) >> pcp->batch = PAGE_SHIFT * 8; >> -- >> 1.8.2 >> > > That is very good. > However, now we've created a "protocol" for updating ->high and ->batch: > > 1. Call pageset_update_prep(pcp) > 2. Update ->high > 3. smp_wmb() > 4. Update ->batch > > But that protocol is not documented anywhere and someone that reads > the code two > years from now will not be aware of it or why it is done like that. > > How about if we create: > > /* > * pcp->high and pcp->batch values are related and dependent on one another: > * ->batch must never be higher then ->high. > * The following function updates them in a safe manner without a > costly atomic transaction. > */ > static void pageset_update(struct per_cpu_pages *pcp, unsigned int > high, unsigned int batch) > { > /* start with a fail safe value for batch */ > pcp->batch = 1; > smp_wmb(); > > /* Update high, then batch, in order */ > pcp->high = high; > smp_wmb(); > pcp->batch = batch; > } > > And use that at the update sites? then the protocol becomes explicit. Oh, and other then that it looks good to me, so assuming you do that - Reviewed-By: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx> Many thanks, Gilad -- Gilad Ben-Yossef Chief Coffee Drinker gilad@xxxxxxxxxxxxx Israel Cell: +972-52-8260388 US Cell: +1-973-8260388 http://benyossef.com "If you take a class in large-scale robotics, can you end up in a situation where the homework eats your dog?" -- Jean-Baptiste Queru -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>