On Sun, Feb 18, 2018 at 09:28:30AM +0200, Andriy Gapon wrote:
A> > A> vnode_pager_getpages_async() at vnode_pager_getpages_async+0x81/frame
A> > A> 0xfffffe00b3c36650
A> > A> vn_sendfile() at vn_sendfile+0xe70/frame 0xfffffe00b3c368e0
A> > A> sendfile() at sendfile+0x149/frame 0xfffffe00b3c36980
A> > A> amd64_syscall() at amd64_syscall+0x79b/frame 0xfffffe00b3c36ab0
A> > A> fast_syscall_common() at fast_syscall_common+0x101/frame 0x7fffffffdb00
A> > A> 
A> > A> I looked at sendfile_swapin() code and it seems that it uses the pager 
API in an
A> > A> undocumented way.  Specifically, it inserts bogus_page into the array of
A> > A> requested pages.  For starters, bogus_page is not busied and 
A> > A> documented to have all requested pages exclusively busied.  Second, I 
always had
A> > A> an impression that bogus_page is an implementation detail of the 
unified buffer
A> > A> / page cache and that other code need not be aware of it.
A> > A> 
A> > A> So, my opinion is that the sendfile code uses a "clever hack" that 
happens to
A> > A> work with the buffer cache based filesystems, but that that hack is a 
A> > A> So, I'd prefer that the problem is fixed in that code.
A> > A> But I am open to being convinced that all VOP_GETPAGES implementations,
A> > A> including that in ZFS, must be made aware of bogus_page.  Or, at least, 
A> > A> they should not verify that the requested pages are busied.
A> > 
A> > This is optimization that improves throughput when file memory cache is
A> > fragmented. Why don't you like adding the code to zfs_freebsd_getpages()?
A> I cited two reasons above and expected to hear some counter-points rather 
A> them being ignored :-)
A> If we settle upon allowing bogus_page to be used in ma[], then that will
A> obviously need to be documented.

My only point is that it is a performance improvement. IMHO that's enough :)
If you can't suggest a more elegant way of doing that improvement, then all
I can suggest is to document it and add its support to ZFS.

Gleb Smirnoff
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