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List:       fuse-devel
Subject:    [fuse-devel] File system cache problem ?
From:       Hans_Beckérus <hans.beckerus () gmail ! com>
Date:       2011-12-20 16:06:24
Message-ID: CAFyqS9r8Yav28BgQ-iNf0-Eajh-ymybfW9G2YYNwASBQb3PwFQ () mail ! gmail ! com
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Hi. I really do not know how to start this, but I have encountered a
problem that has bugged me forever.
I have turned my file system inside out for bugs but I can not find
anything really that indicates a problem in my code.
I will try to explain the problem. Hopefully it will make sense.

My file system handles compressed streams of data. This means that in
some specific situations (depending on what tool is used) when an
offset is requested
far outside current stream position I am forced to return some dummy
data, still according to the size as specified in the request. In this
case I just return a buffer filled with 0xff.
If I at the same time just do 'cat' on the file problem starts :(
'cat' is just reading the file sequentially, so there is no need to
fake data in this case.
However, if I first run 'cat' on the file and while it is working,
start another tool that will access the same file but forces my file
system to return the dummy buffer, the contents of the 'cat' redirect
output is corrupted! The very same dummy 0xff buffer can be found by
doing hexdump on the output!
What I can see tell by adding traces in my code is that the read
offset from where the dummy buffer was returned is never entering my
file system for the 'cat':ed file! Hence, my guess is that is was
picked up from a cache "somewhere". I do not set fi->keep_cache,
actually I force it to 0 at each open request but that still does not
seem to help.  Also, I can not for my life understand why doing it in
the opposite order works!? If I first start the tool that forces me to
return a dummy buffer, and after that buffer has been returned do the
'cat' operation, it works!??

Is there some way in which I can force a flush of the file system
cache for each open of a file to force it to enter my callbacks at all
times? The keep_cache=0 does not seem to help, also from what I
understand the value of 0 in this case is the default. If I set it to
1 problem gets even worse.
I was thinking about the O_SYNC flag in open, but is that really an
option here? I can not tell the user of the file system how to open a
file using standard tools like 'cat' :(

Any ideas what I could try next?

Hans

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