linux-kernel - Re: Should implementations of ->direct

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Message-ID: <5517B18A.3050305@plexistor.com>
Date:	Sun, 29 Mar 2015 11:02:18 +0300
From:	Boaz Harrosh <boaz@...xistor.com>
To:	Dave Chinner <david@...morbit.com>,
	Matthew Wilcox <willy@...ux.intel.com>
CC:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, msharbiani@...pensource.com
Subject: Re: Should implementations of ->direct_access be allowed to sleep?

On 03/26/2015 09:32 PM, Dave Chinner wrote:
<>
>> I'm leaning towards the latter.  But I'm not sure what GFP flags to
>> recommend that brd use ... GFP_NOWAIT | __GFP_ZERO, perhaps?
> 
> What, so we get random IO failures under memory pressure?
> 
> I really think we should allow .direct_access to sleep. It means we
> can use existing drivers and it also allows future implementations
> that might require, say, RDMA to be performed to update a page
> before access is granted. i.e. .direct_access is the first hook into
> the persistent device at page fault time....
> 

I agree with Dave. Last I tried (couple years ago) doing any
allocation GFP_NOWAIT on FS IO paths fails really badly in all kind
of surprising ways. The Kernel is built in to that allocation pressure.

I think that ->direct_access should not be any different then
any other block-device access, ie allow to sleep.

With brd a user can make sure not to sleep if he pre-allocates
ie call ->direct_access at least once on a given offset-length.
But I would not like to even do that guaranty. ->direct_access
should be allowed to sleep.
Well written code has many ways to allow sleep yet be very low
latency. (So I do not see what we are missing)

> Cheers,
> Dave.

Thanks
Boaz

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