Messages in this thread | | | Subject | Re: [PATCH] cowlinks v2 | From | (Eric W. Biederman) | Date | 03 Apr 2004 13:30:24 -0700 |
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Jamie Lokier <jamie@shareable.org> writes:
> Eric W. Biederman wrote: > > > Here's a tricky situation: > > > > > > 1. A file is cowlinked. Then each cowlink is mmap()'d, one per process. > > > > > > 2. At this point both mappings share the same pages in RAM. > > > > Why they have different inodes? > > Did you miss the last 20 or so messages in this thread? > > We'd like cowlinks that are an invisible filesystem optimisation. > That means you "copy" a file and it behaves the same as if you copy a file.
Exactly so they would not share the same pages in RAM.
> > > 3. Then one of the cowlinks is written to... > > > > I would not worry about sharing page cache entries unless this becomes > > a common case. If you want to avoid the hit of rereading the file when > > you have a cow copy it should be simple enough to walk through the list > > of cow copies and see if anyone else has it open. > > It is not a question of performance, it's correctness. Either you > have cowlinks that behave like copied files, or you accept that when > cowlinked files are mmapped and written to, they don't behave like > regular files (not even the original file prior to cowlinking does). > > Btw, I'm not suggesting sharing page cache entries.
It sounded like you assumed sharing of page cache entries above. How do you get to step 2 if the cow copies don't share the same page cache entries?
Eric
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