[Bug tapsets/22857] New: vfs.do_mpage_readpage doesn't work as expected on ext4 ilesystems

["evan at eklitzke dot org" <sourceware-bugzilla at sourceware dot org>]

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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