Messages in this thread |  | | Subject | Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected | From | Jürgen Groß <> | Date | Mon, 16 Dec 2019 17:23:44 +0100 |
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On 16.12.19 17:15, SeongJae Park wrote: > On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park <sjpark@amazon.com> wrote: > >> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park <sjpark@amazon.com> wrote: >> >>> From: SeongJae Park <sjpark@amazon.de> >>> > [...] >>> --- a/drivers/block/xen-blkback/xenbus.c >>> +++ b/drivers/block/xen-blkback/xenbus.c >>> @@ -824,6 +824,24 @@ static void frontend_changed(struct xenbus_device *dev, >>> } >>> >>> >>> +/* Once a memory pressure is detected, squeeze free page pools for a while. */ >>> +static unsigned int buffer_squeeze_duration_ms = 10; >>> +module_param_named(buffer_squeeze_duration_ms, >>> + buffer_squeeze_duration_ms, int, 0644); >>> +MODULE_PARM_DESC(buffer_squeeze_duration_ms, >>> +"Duration in ms to squeeze pages buffer when a memory pressure is detected"); >>> + >>> +/* >>> + * Callback received when the memory pressure is detected. >>> + */ >>> +static void reclaim_memory(struct xenbus_device *dev) >>> +{ >>> + struct backend_info *be = dev_get_drvdata(&dev->dev); >>> + >>> + be->blkif->buffer_squeeze_end = jiffies + >>> + msecs_to_jiffies(buffer_squeeze_duration_ms); >> >> This callback might race with 'xen_blkbk_probe()'. The race could result in >> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links >> 'be' to the 'dev'. Please _don't merge_ this patch now! >> >> I will do more test and share results. Meanwhile, if you have any opinion, >> please let me know. > > Not only '->blkif', but 'be' itself also coule be a NULL. As similar > concurrency issues could be in other drivers in their way, I suggest to change > the reclaim callback ('->reclaim_memory') to be called for each driver instead > of each device. Then, each driver could be able to deal with its concurrency > issues by itself.
Hmm, I don't like that. This would need to be changed back in case we add per-guest quota.
Wouldn't a get_device() before calling the callback and a put_device() afterwards avoid that problem?
Juergen
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