Re: [Qemu-devel] [PATCH v19 3/7] xbitmap: add more operations

[Tetsuo Handa]

Wei Wang wrote:
> On 12/16/2017 02:42 AM, Matthew Wilcox wrote:
> > On Tue, Dec 12, 2017 at 07:55:55PM +0800, Wei Wang wrote:
> >> +int xb_preload_and_set_bit(struct xb *xb, unsigned long bit, gfp_t gfp);
> > I'm struggling to understand when one would use this.  The xb_ API
> > requires you to handle your own locking.  But specifying GFP flags
> > here implies you can sleep.  So ... um ... there's no locking?
> 
> In the regular use cases, people would do xb_preload() before taking the 
> lock, and the xb_set/clear within the lock.
> 
> In the virtio-balloon usage, we have a large number of bits to set with 
> the balloon_lock being held (we're not unlocking for each bit), so we 
> used the above wrapper to do preload and set within the balloon_lock, 
> and passed in GFP_NOWAIT to avoid sleeping. Probably we can change to 
> put this wrapper implementation to virtio-balloon, since it would not be 
> useful for the regular cases.

GFP_NOWAIT is chosen in order not to try to OOM-kill something, isn't it?
But passing GFP_NOWAIT means that we can handle allocation failure. There is
no need to use preload approach when we can handle allocation failure.