Linus Torvalds writes:
>
[...]
>
> Now, admittedly it has a few downsides:
>
> - right now "readpage()" is called in several places, and you'd have to
> create some kind of nice wrapper for the most common
> "mapping->a_ops->readpage()" thing and hook into there to avoid
> duplicating the effort.
>
> Alternatively, you could decide that you only want to do this at the
> filesystem level, which actually simplifies some things. If you
> instrument "mpage_readpage[2]()", you'll already get several of the
> ones you care about, and you could do the others individually.
>
> [ As a third alternative, you might decide that the only thing you
> actually care about is when you have to wait on a locked page, and
> instrument the page wait-queues instead. ]
>
> - it will miss any situation where a filesystem does a read some other
> way. Notably, in many loads, the _directory_ accesses are the important
> ones, and if you want statistics for those you'd often have to do that
> separately (not always - some of the filesystems just use the same
> page reading stuff).
Another disadvantage is that ->readpage() can only do read-ahead within
single file, which is not helpful for the case of reading a lot of small
files (and this is what happens during startup).
And to implement reasonable multi-file read-ahead at the file system
layer one needs asynchronous inode loading interface implemented for
every file system.
Nikita.
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