nfs pessimized by vnode pages changes

[Gleb Smirnoff]

On Tue, Mar 29, 2016 at 01:24:27PM +1100, Bruce Evans wrote:
B> > B> I couldn't get full-fs-block input to work right in ncl_getpages().  In
B> > B> the old version, vm_fault_hold() almost always calls it with the pages
B> > B> for exactly 1 full block, because vm_fault_additional_pages() somehow
B> > B> finds this many pages.
B> >
B> > The last quoted paragraph is a correct observation. According to your
B> > investigations, prior to r292373 NFS was doing multiple page pageins,
B> > despite it should have reported that it can't. My reading of the code
B> > is the same: both before r292373 and after r292373 NFS should page in
B> > a single page at request. I quickly reviewed the whole codepath and I
B> > can't see how with older code it was able to do multiple page pageins.
B> 
B> I found it.  In old versions, vm_fault_additional_pages() exists and
B> calls vnode_pager_haspage() to locate some additional pages.
B> VOP_BMAP() is vop_stdbmap() for nfs.  This doesn't locate any
B> additional fs blocks, but vnode_pager_haspage() locates the page within
B> the single nfs block and returns the number of pages to read behind and
B> ahead to fill in the block.

Hmm, the code in vnode_pager_generic_getpages() after my change does pretty
much the same:

        /* Recalculate blocks available after/before to pages. */
        poff = (foff % bsize) / PAGE_SIZE;
        before *= pagesperblock;
        before += poff;
        after *= pagesperblock;   
        after += pagesperblock - (poff + 1);

So, we got zeroes in 'after' and 'before' after vop_stdbmap. But here we
add 'poff' to them, which should yield in the same read sizes as before.

I will debug that.

-- 
Totus tuus, Glebius.