Does VFS cache individual files? Is the linker involved?

Does VFS cache individual files? Is the linker involved?

[Magnus Sundberg]

Dear sirs,

QUESTION:
Does VFS cache the content of a file?

Does VFS consult memory instead of disk, when accessing a file that is
already linked to a process that is running?


RELATED INFORMATION:
Operating system RedHat 7.1, kernel 2.4.3-12 (from RedHat RPM)
My webserver runs tripwire for integrity test of critical files.

One night, when the PHP runtime of apache had crashed, tripwire reported
checksum error of
/usr/lib/apache/libphp4.so
The next night, when apache was restarted, there where no checksum
errors of the file.

I have seen checksum errors of /lib/libc-2.2.2.so, rpm also reported md5
checksum error when I
used rpm to verify the installation of glibc.

Thank you for your time reading this

regards,

Magnus Sundberg

Re: Does VFS cache individual files? Is the linker involved?

[Alan Cox]

> Does VFS cache the content of a file?

It caches arbitary pages of the file.

> Does VFS consult memory instead of disk, when accessing a file that is
> already linked to a process that is running?

Depends if the block is already in memory. If it is paged in then it will
used a shared read only copy when there is one. If not then it will load it

> One night, when the PHP runtime of apache had crashed, tripwire reported
> checksum error of
> /usr/lib/apache/libphp4.so
> The next night, when apache was restarted, there where no checksum
> errors of the file.
> 
> I have seen checksum errors of /lib/libc-2.2.2.so, rpm also reported md5
> checksum error when I
> used rpm to verify the installation of glibc.

That is consistent with in memory file cache corruption yes. Since the error
never got written back to disk (and its probably in a clean page) it
vanished when the cached copy did