[issue38167] O_DIRECT read fails with 4K mmap buffer

[Josh Rosenberg]

Josh Rosenberg  added the comment:

Yeah, not a bug. The I/O subsystem was substantially rewritten between Python 2 
and Python 3, so you sometimes need to be more explicit about things like 
buffering, but as you note, once the buffering is correct, the code works; 
there's nothing to fix.

--
resolution:  -> not a bug
stage: patch review -> resolved
status: open -> closed

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Paul]

Paul  added the comment:

> Problem is you follow it with:
>
> fo = os.fdopen(fd, 'rb+')

> which introduces a Python level of buffering around the kernel unbuffered 
> file descriptor. You'd need to pass buffering=0 to make os.fdopen avoid 
> returning a buffered file object, making it:

> fo = os.fdopen(fd, 'rb+', buffering=0)

You are absolutely right!  This fixed the issue.  So... is this not a bug, 
then?  Should I discard my patch?

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Josh Rosenberg]

Josh Rosenberg  added the comment:

> I do not believe an unbuffered file uses O_DIRECT.  This is why I use 
> os.open(fpath, os.O_DIRECT).

Problem is you follow it with:

fo = os.fdopen(fd, 'rb+')

which introduces a Python level of buffering around the kernel unbuffered file 
descriptor. You'd need to pass buffering=0 to make os.fdopen avoid returning a 
buffered file object, making it:

fo = os.fdopen(fd, 'rb+', buffering=0)

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Paul]

Paul  added the comment:

> I agree with Josh. If you want to use O_DIRECT, use an unbuffered file object 
> and be sure to issue reads of the right size.

I do not believe an unbuffered file uses O_DIRECT.  This is why I use 
os.open(fpath, os.O_DIRECT).

> Also I'm curious: why are you using O_DIRECT, and furthermore, why are you 
> using it to read into mmap'ed memory?

I am testing a storage device and must use O_DIRECT to avoid the kernel's 
cache.  I am using mmap because it was the simplest way to get a page-aligned 
memory buffer, which is required for direct IO.

I believe that this is a bug regardless of the use of mmap, especially 
considering that this worked in Python 2.  I believe the fix I have sent out 
for review addresses it adequately.

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Also I'm curious: why are you using O_DIRECT, and furthermore, why are you 
using it to read into mmap'ed memory?

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Antoine Pitrou]

Antoine Pitrou  added the comment:

I agree with Josh. If you want to use O_DIRECT, use an unbuffered file object 
and be sure to issue reads of the right size.

--
nosy: +pitrou

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Paul]

Paul  added the comment:

This is the platform that I'm working on as well as the failure.  I have a 
review out for a fix.

# uname -a
Linux init129-13 3.10.0-957.el7.x86_64 x86_64 x86_64 x86_64 GNU/Linux
# python3.7 directread.py
Traceback (most recent call last):
  File "small.py", line 7, in 
fo.readinto(m)
OSError: [Errno 22] Invalid argument

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Josh Rosenberg]

Josh Rosenberg  added the comment:

Works just fine for me on 3.7.3 on Ubuntu, reading 4096 bytes. How is it 
failing for you? Is an exception raised?

It does seem faintly dangerous to explicitly use O_DIRECT when you're wrapping 
it in a buffered reader that doesn't know it has to read in units matching the 
minimum block size (file system dependent on older kernels, 512 bytes in Linux 
kernel 2.6+); BufferedIOBase.readinto is explicitly documented to potentially 
issue multiple read calls (readinto1 guarantees it won't do that at least).

--
nosy: +josh.r

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Paul]

Change by Paul :


--
pull_requests: +15742
pull_request: https://github.com/python/cpython/pull/16131

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Paul]

Change by Paul :


--
keywords: +patch
pull_requests: +15741
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/16130

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[issue38167] O_DIRECT read fails with 4K mmap buffer

[Paul]

New submission from Paul :

The following Python3 script fails.

import mmap
import os

fd = os.open(path_to_file, os.O_DIRECT | os.O_RDWR)
fo = os.fdopen(fd, 'rb+')
m = mmap.mmap(-1, 4096)
fo.readinto(m)

But it worked for Python2.  It also works for any other multiple of 4K. For 
example:

m = mmap.mmap(-1, 8192)
fo.readinto(m)

Is fine!

--
components: IO
messages: 352397
nosy: yoyoyopcp
priority: normal
severity: normal
status: open
title: O_DIRECT read fails with 4K mmap buffer
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8, Python 3.9

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