[Bug 246886] [sendfile] Nginx + NFS or FUSE causes VM stall

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246886

--- Comment #54 from Gleb Smirnoff <glebius at FreeBSD.org> ---
Then that's a crazy mix of subsystems involved!

Here goes description of how nginx combines aio and sendfile. When sendfile(2)
returns EBUSY, nginx would do aio_read(2) of 1 byte out of the file, and use a
side effect of the page being cached, then it would retry sendfile again.

Note, that EBUSY from sendfile in FreeBSD 11 and before means totally different
thing than EBUSY from sendfile on FreeBSD 12 and after. On older versions EBUSY
means that SF_NODISKIO was set on request and file is not in memory. On new
versions EBUSY is a soft error that means page is busy (again SF_NODISKIO is
set). The busy condition would go away in a few milliseconds. However, nginx
doesn't differentiate between versions of FreeBSD and always does this
aio_read() of 1 byte. It is harmless on newer versions of FreeBSD and just adds
a small delay before retry. At Netflix we have removed this code from nginx and
in case of EBUSY we just retry after a timer, so we don't use aio.

It is entirely possible that problem shows up only when aio_read() and sendfile
are combined on the same region of a file on NFS or FUSE :(

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