Re: [PATCH v3] xfs: assure zeroed memory buffers for certain kmem allocations

From: Dave Chinner <david@fromorbit.com>
To: Eric Sandeen <sandeen@sandeen.net>
Cc: Christoph Hellwig <hch@infradead.org>,
	Bill O'Donnell <billodo@redhat.com>,
	linux-xfs@vger.kernel.org
Subject: Re: [PATCH v3] xfs: assure zeroed memory buffers for certain kmem allocations
Date: Tue, 24 Sep 2019 14:13:32 +1000	[thread overview]
Message-ID: <20190924041332.GE16973@dread.disaster.area> (raw)
In-Reply-To: <dbf196da-bede-6bbe-db3e-35d7040170d0@sandeen.net>

On Fri, Sep 20, 2019 at 09:59:41AM -0500, Eric Sandeen wrote:
> On 9/19/19 12:38 PM, Christoph Hellwig wrote:
> > On Thu, Sep 19, 2019 at 12:20:47PM -0500, Bill O'Donnell wrote:
> >>>> @@ -391,7 +396,7 @@ xfs_buf_allocate_memory(
> >>>>  		struct page	*page;
> >>>>  		uint		retries = 0;
> >>>>  retry:
> >>>> -		page = alloc_page(gfp_mask);
> >>>> +		page = alloc_page(gfp_mask | kmflag_mask);
> >>>
> >>> alloc_page takes GFP_ flags, not KM_.  In fact sparse should have warned
> >>> about this.
> >>
> >> I wondered if the KM flag needed conversion to GFP, but saw no warning.
> > 
> > I'd be tempted to just do a manual memset after either kind of
> > allocation.
> 
> At some point I think Dave had suggested that at least when allocating pages,
> using the flag would be more efficient?

With some configurations pages come from the free lists pre-zeroed,
and so don't need zeroing to initialise them (e.g. when memory
poisoning is turned on, or pages are being zeroed on free). Hence if
you use __GFP_ZERO the it will only zero if the page obtained from
the freelist isn't already zero. The __GFP_ZERO call will also use
the most efficient method of zeroing the page for the platform via
clear_page() rather than memset()....

/me shrugs and doesn't really care either way....

-Dave.
-- 
Dave Chinner
david@fromorbit.com