Re: [PATCH] update ctime and mtime for mmaped write

From: Peter Staubach
Date: Thu Feb 22 2007 - 12:53:50 EST

Miklos Szeredi wrote:
Inspired by Peter Staubach's patch and the resulting comments.

An updated version of the original patch was submitted to LKML
yesterday... :-)
Strange coincidence :)

file = vma->vm_file;
start = vma->vm_end;
+ mapping_update_time(file);
if ((flags & MS_SYNC) && file &&
(vma->vm_flags & VM_SHARED)) {
It seems to me that this might lead to file times being updated for
non-MAP_SHARED mappings.
In theory no, because the COW-ed pages become anonymous and are not
part of the original mapping any more.

I must profess to having a incomplete understanding of all of this
support, but then why would it be necessary to test VM_SHARED at
this point in msync()?

That's basically just an optimization. If it wasn't there, then data
from a another (shared) mapping could be written back, which is not
wrong, but not required either.

I ran into problems early on with file times being updated incorrectly
so I am a little sensitive this aspect.

+int set_page_dirty_mapping(struct page *page);
This aspect of the design seems intrusive to me. I didn't see a strong
reason to introduce new versions of many of the routines just to handle
these semantics. What motivated this part of your design? Why the new
_mapping versions of routines?
Because there's no way to know inside the set_page_dirty() functions
if the dirtying comes from a memory mapping or from a modification
through a normal write(). And they have different semantics, for
write() the modification times are updated immediately.
Perhaps I didn't understand what page_mapped() does, but it does seem to
have the right semantics as far as I could see.

The problems will start, when you have a file that is both mapped and
modified with write(). Then the dirying from the write() will set the
flag, and that will have undesirable consequences.

I don't think that I quite follow the logic. The dirtying from write()
will set the flag, but then the mtime will get updated and the flag will
be cleared by the hook in file_update_time(). Right?


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