Think about it:-) You need to generate prefetch accesses in ascending
device bnum order. So the bmap() is there to tell you those device
bnums. You'd still prefetch using file bnums, the the *ordering* is
done based on device bnum. In fact, once the list is sorted, you can
chuck out the device bnums. You only need to store inum/path and file
bnum in the database.
> > One approach would be to create a new ioctl(2) for a FS that would
> > read out inum,bnum pairs.
> Why not just "path,pagenr" instead? You make your instrumentation save
> away the whole pathname, by just using the dentry pointer. Many
> filesystems don't even _have_ a "inum", so anything less doesn't work
Sure, this would work too. I'm a bit worried about the increased
amount of traffic this will generate.
> Example acceptable approach:
> - save away full dentry and page number. Don't make it an ioctl. Think
> "profiling" - this is _exactly_ the same thing, and profiling uses a
> (a) command line argument to turn it on
> (b) /proc/profile
> (and because you have the full pathname, you should just make the dang
> /proc/fsaccess file be ASCII)
So on every page fault or read(2) call, we have to generate the full
path from the dentry? Isn't that going to add a fair bit of overhead?
Remember, we want to do this on every boot (to keep the database as
up-to-date as possible).
> - add a "prefetch()" system call that does all the same things
> "read()" does, but doesn't actually wait for (or transfer) the
> data. Basically just a read-ahead thing. So you'd basically end up
> foreach (filename in /proc/fsaccess)
> fd = open(filename);
> foreach (sorted pagenr for filename in /proc/fsaccess)
> prefetch(fd, pagenr);
I don't see the advantage of the prefetch(2) system call. It seems to
me I can get the same effect by just making read(2) calls in another
task. Of course, I'd need to use bmap() to generate the sort key, but
I don't see why that's a bad thing.
> Forget about all these crappy "ioctl" ideas. Basic rule of thumb: if
> you think an ioctl is a good idea, you're (a) being stupid and (b)
> thinking wrong and (c) on the wrong track.
Don't hold back now. Tell us what you really think :-)
> And notice how there's not a single bmap anywhere, and not a single
> "raw device open" anywhere.
I don't mind the /proc/fsaccess approach, I'm just worried about the
overhead of doing the denty->pathname conversions on each fault/read.
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