Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)

[Milosz Tanski <milosz@adfin.com>]

Jeff,

This patchset creates a new read (readv2/preadv2) syscall(s) that take
a extra flag argument (kind of like recvmsg). What it doesn't do is
change the current behavior of of the O_NONBLOCK, if the file is
open() with O_NONBLOCK flag. It shouldn't break any existing
applications since you have to opt into using this by using the new
syscall.

I don't have a preference either way if we should create a new flag or
re-use O_NONBLOCK the flag. Instead, I'm hoping to get some consensus
here from senior kernel developers like yourself. Maybe a RWF_NONBLOCK
(I'm stealing from eventfd, EFD_NONBLOCK).

As a side note, I noticed that EFD_NONBLOCK, SFD_NONBLOCK, etc... all
alias to the value of O_NONBLOCK and there's a bunch of bug checks in
the code like this:
BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);

Thanks,
- Milosz

On Mon, Sep 15, 2014 at 5:58 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> Hi, Milosz,
>
> I CC'd Michael Kerrisk, in case he has any opinions on the matter.
>
> Milosz Tanski <milosz@adfin.com> writes:
>
>> This patcheset introduces an ability to perform a non-blocking read from
>> regular files in buffered IO mode. This works by only for those filesystems
>> that have data in the page cache.
>>
>> It does this by introducing new syscalls new syscalls readv2/writev2 and
>> preadv2/pwritev2. These new syscalls behave like the network sendmsg, recvmsg
>> syscalls that accept an extra flag argument (O_NONBLOCK).
>
> I thought you were going to introduce a new flag instead of using
> O_NONBLOCK for this.  I dug up an old email that suggested that enabling
> O_NONBLOCK for regular files (well, a device node in this case) broke a
> cd ripping or burning application.  I also found this old bugzilla,
> which states that squid would fail to start, and that gqview was also
> broken:
>   https://bugzilla.redhat.com/show_bug.cgi?id=136057
>
> More generally, do you expect the open(2) of a regular file with
> O_NONBLOCK to perform the same way as a pipe, fifo, or device (namely,
> that the open itself won't block)?  Should O_NONBLOCK affect writes to
> regular files?  What do you think the return value from poll and friends
> should be when a file is opened in this manner (probably not important,
> as poll always returns data ready on regular files)?  Also consider
> whether you want the O_NONBLOCK behaviour for mandatory file locks in
> your use case (or any other, for that matter).  If you issue a read and
> it returns -EAGAIN, should it be up to the application to kick off I/O
> to ensure it makes progress?
>
> I don't think O_NONBLOCK is the right flag.  What you're really
> specifying is a flag that prevents I/O in the read path, and nowhere
> else.  As such, I'd feel much better about this if we defined a new flag
> (O_NONBLOCK_READ maybe?  No, that's too verbose.).
>
> In summary, I like the idea, but I worry about overloading O_NONBLOCK.
>
> Cheers,
> Jeff



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Milosz Tanski
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