[Bug 222356] www/firefox: file-backed shared memory performance

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222356

--- Comment #3 from Konstantin Belousov <kib at FreeBSD.org> ---
(In reply to Tijl Coosemans from comment #2)

Which file system do you use ?  The supposed mechanism is that when last
on-disk link goes away, vp->v_vflag gets VV_NOSYNC bit set.  Look for instance
at the ufs_remove().  Then, vm_object_page_clean() skips such vnodes, and this
function is exactly what syncer calls to clean vnode' cached pages.

How did you ensured that there is an io caused by the write-out of the data ? 
The metadata must be written on the unmap, and even unlinked vnode removal
causes a lot of metadata writes on UFS.  The inode must be prepared for reuse
by writing initial data, all inode data blocks must be marked as free in the
cylinder group bitmask, the inode must be marked as free in another bitmask,
then cg summary must be updated indicating new amount of free blocks and
inodes.

As a side note, I do not see how could my patch cause failures which you
described.  All it does is preventing msync(2) from actually write pages to
disk for the unlinked file.

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