* [Bug tapsets/22857] New: vfs.do_mpage_readpage doesn't work as expected on ext4 ilesystems
@ 2018-02-18 5:50 evan at eklitzke dot org
2018-02-18 5:52 ` [Bug tapsets/22857] vfs.do_mpage_readpage doesn't work as expected on ext4 filesystems evan at eklitzke dot org
2018-02-23 19:17 ` dsmith at redhat dot com
0 siblings, 2 replies; 3+ messages in thread
From: evan at eklitzke dot org @ 2018-02-18 5:50 UTC (permalink / raw)
To: systemtap
https://sourceware.org/bugzilla/show_bug.cgi?id=22857
Bug ID: 22857
Summary: vfs.do_mpage_readpage doesn't work as expected on ext4
ilesystems
Product: systemtap
Version: unspecified
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: tapsets
Assignee: systemtap at sourceware dot org
Reporter: evan at eklitzke dot org
Target Milestone: ---
The VFS tapset defines vfs.do_mpage_readpage which probes the kernel function
of the same name. This probe point would typically be used by Systemtap scripts
that want to collect information about disk activity occurring as the result of
accessing data in an mmaped file.
However, ext4 specializes this behavior, and provides its own
ext4_mpage_readpages method which is used in the same context (i.e. when
submitting bio requests for filed-backed mmap pages), and the Systemtap VFS
tapset doesn't probe this. This means that vfs.do_mpage_readpage is effectively
broken for a large percentage of Linux users.
This is a little bit trick to fix because these two methods have different
interfaces. I think it might be possible to refactor the Linux code for
ext4_mpage_readpages to call a do_ method that has the same type signature as
do_mpage_readpage, but that would require a change to the Linux kernel. In the
meantime it might make sense to add a new vfs probe point just for
ext4_mpage_readpages.
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* [Bug tapsets/22857] vfs.do_mpage_readpage doesn't work as expected on ext4 filesystems
2018-02-18 5:50 [Bug tapsets/22857] New: vfs.do_mpage_readpage doesn't work as expected on ext4 ilesystems evan at eklitzke dot org
@ 2018-02-18 5:52 ` evan at eklitzke dot org
2018-02-23 19:17 ` dsmith at redhat dot com
1 sibling, 0 replies; 3+ messages in thread
From: evan at eklitzke dot org @ 2018-02-18 5:52 UTC (permalink / raw)
To: systemtap
https://sourceware.org/bugzilla/show_bug.cgi?id=22857
Evan Klitzke <evan at eklitzke dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|vfs.do_mpage_readpage |vfs.do_mpage_readpage
|doesn't work as expected on |doesn't work as expected on
|ext4 ilesystems |ext4 filesystems
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* [Bug tapsets/22857] vfs.do_mpage_readpage doesn't work as expected on ext4 filesystems
2018-02-18 5:50 [Bug tapsets/22857] New: vfs.do_mpage_readpage doesn't work as expected on ext4 ilesystems evan at eklitzke dot org
2018-02-18 5:52 ` [Bug tapsets/22857] vfs.do_mpage_readpage doesn't work as expected on ext4 filesystems evan at eklitzke dot org
@ 2018-02-23 19:17 ` dsmith at redhat dot com
1 sibling, 0 replies; 3+ messages in thread
From: dsmith at redhat dot com @ 2018-02-23 19:17 UTC (permalink / raw)
To: systemtap
https://sourceware.org/bugzilla/show_bug.cgi?id=22857
David Smith <dsmith at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |dsmith at redhat dot com
Assignee|systemtap at sourceware dot org |dsmith at redhat dot com
--- Comment #1 from David Smith <dsmith at redhat dot com> ---
Created attachment 10847
--> https://sourceware.org/bugzilla/attachment.cgi?id=10847&action=edit
proposed patch
Here's a patch where I try to hook ext4_mpage_readpages() into the
vfs.do_mpage_readpage probe alias. It compiles, but I haven't really tested it.
Can you test it and let me know if it works for you?
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2018-02-18 5:50 [Bug tapsets/22857] New: vfs.do_mpage_readpage doesn't work as expected on ext4 ilesystems evan at eklitzke dot org
2018-02-18 5:52 ` [Bug tapsets/22857] vfs.do_mpage_readpage doesn't work as expected on ext4 filesystems evan at eklitzke dot org
2018-02-23 19:17 ` dsmith at redhat dot com
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