On Thu, Oct 16, 2014 at 12:20:47PM +0200, Mathieu Desnoyers wrote:
> > +/*
> > + * The user has performed a load from a hole in the file. Allocating
> > + * a new page in the file would cause excessive storage usage for
> > + * workloads with sparse files. We allocate a page cache page instead.
> > + * We'll kick it out of the page cache if it's ever written to,
> > + * otherwise it will simply fall out of the page cache under memory
> > + * pressure without ever having been dirtied.
>
> Nice trick :)
It's basically what the page cache does. Unfortunately, I had to step
out of the room while Calvin detailed his trick for doing it differently,
but if his patch goes in, we should follow suit.
> > + if (!page) {
> > + mutex_lock(&mapping->i_mmap_mutex);
> > + /* Check we didn't race with truncate */
> > + size = (i_size_read(inode) + PAGE_SIZE - 1) >>
> > + PAGE_SHIFT;
> > + if (vmf->pgoff >= size) {
> > + mutex_unlock(&mapping->i_mmap_mutex);
> > + error = -EIO;
> > + goto out;
> > + }
> > + }
>
> If page is non-NULL, is it possible that we return VM_FAULT_LOCKED
> without actually holding i_mmap_mutex ? Is it on purpose ?
>
> > + return VM_FAULT_LOCKED;
> > + }
That's right; this is the original meaning of VM_FAULT_LOCKED, that the
page lock is held. We took it before the call to get_block(), ensuring
that we don't hit the truncate race. Er ... hang on. At some point in
the revising of patches, I dropped the stanza where we re-check i_size
after grabbing the page lock. Sod ... a v12 of this patchset will have
to be forthcoming!
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