lkml.org 
[lkml]   [2014]   [Oct]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] aio: Fix return code of io_submit() (RFC)
    On 2014-10-03 12:21, Kent Overstreet wrote:
    > On Fri, Oct 03, 2014 at 12:13:39PM -0600, Jens Axboe wrote:
    >> On 2014-10-03 12:08, Kent Overstreet wrote:
    >>> io_submit() could return -EAGAIN on memory allocation failure when it should
    >>> really have been returning -ENOMEM. This could confuse applications (i.e. fio)
    >>> since -EAGAIN means "too many requests outstanding, wait until completions have
    >>> been reaped" and if the application actually was tracking outstanding
    >>> completions this wouldn't make a lot of sense.
    >>>
    >>> NOTE:
    >>>
    >>> the man page seems to imply that the current behaviour (-EAGAIN on allocation
    >>> failure) has always been the case. I don't think it makes a lot of sense, but
    >>> this should probably be discussed more widely in case applications have somehow
    >>> come to rely on the current behaviour...
    >>
    >> We can't really feasibly fix this, is my worry. Fio does track the pending
    >> requests and does not get into a getevents() forever wait if it gets -EAGAIN
    >> on submission. But before the fix, it would loop forever in submission in
    >> -EAGAIN.
    >>
    >> How are applications supposed to deal with ENOMEM? I think the answer here
    >> is that they can't, it would be a fatal condition. AIO must provide isn't
    >> own guarantee of progress, with a mempool or similar.
    >
    > Well, even though the AIO code doesn't currently return -ENOMEM we definitely do
    > have random other driver/filesystem code that will return -ENOMEM if a random
    > GFP_KERNEL allocation fails (e.g. the dio code, if allocating a struct dio
    > fails). So I think there's precedent for this, and having it be a fatal error
    > when the system is under major memory pressure is not a crazy thing to do too.
    >
    > But OTOH maybe we should just use a mempool there.
    >
    > The argument against making it a mempool would be "we don't want io_submit() to
    > block; even if that's not the case today, we at least have a chance of fixing it
    > with the current setup. If we can't allocate memory for our asynchronous state,
    > we really can't do anything there except block or fail".

    It'll block anyway in other places, if we run out of resources there.
    But good point on the other potential -ENOMEM cases, it's not a new
    condition (potentially).

    > I'm not sure I have strong feelings one way or the other.

    Me neither...

    --
    Jens Axboe



    \
     
     \ /
      Last update: 2014-10-03 21:01    [W:2.788 / U:2.084 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site