lkml.org 
[lkml]   [2015]   [Mar]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: 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



\
 
 \ /
  Last update: 2015-03-29 10:41    [W:0.085 / U:0.024 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site