Re: [PATCH v4 0/6] net/tap: Fix QEMU frozen issue when the maximum number of file descriptors is very large

[Markus Armbruster <armbru@redhat.com>]

Jason Wang <jasowang@redhat.com> writes:

> On Sun, Jul 9, 2023 at 11:48 PM Bin Meng <bmeng.cn@gmail.com> wrote:
>>
>> On Wed, Jun 28, 2023 at 11:29 PM Bin Meng <bmeng@tinylab.org> wrote:
>> >
>> >
>> > Current codes using a brute-force traversal of all file descriptors
>> > do not scale on a system where the maximum number of file descriptors
>> > is set to a very large value (e.g.: in a Docker container of Manjaro
>> > distribution it is set to 1073741816). QEMU just looks frozen during
>> > start-up.
>> >
>> > The close-on-exec flag (O_CLOEXEC) was introduced since Linux kernel
>> > 2.6.23, FreeBSD 8.3, OpenBSD 5.0, Solaris 11. While it's true QEMU
>> > doesn't need to manually close the fds for child process as the proper
>> > O_CLOEXEC flag should have been set properly on files with its own
>> > codes, QEMU uses a huge number of 3rd party libraries and we don't
>> > trust them to reliably be using O_CLOEXEC on everything they open.
>> >
>> > Modern Linux and BSDs have the close_range() call we can use to do the
>> > job, and on Linux we have one more way to walk through /proc/self/fd
>> > to complete the task efficiently, which is what qemu_close_range()
>> > does, a new API we add in util/osdep.c.
>> >
>> > V1 link: https://lore.kernel.org/qemu-devel/20230406112041.798585-1-bmeng@tinylab.org/
>> >
>> > Changes in v4:
>> > - add 'first > last' check logic
>> > - reorder the ifdefs logic
>> > - change i to unsigned int type
>> > - use qemu_strtoi() instead of atoi()
>> > - limit last upper value to sysconf(_SC_OPEN_MAX) - 1
>> > - call sysconf directly instead of using a variable
>> > - put fd on its own line
>> >
>> > Changes in v3:
>> > - fix win32 build failure
>> > - limit the last_fd of qemu_close_range() to sysconf(_SC_OPEN_MAX)
>> >
>> > Changes in v2:
>> > - new patch: "tests/tcg/cris: Fix the coding style"
>> > - new patch: "tests/tcg/cris: Correct the off-by-one error"
>> > - new patch: "util/async-teardown: Fall back to close fds one by one"
>> > - new patch: "util/osdep: Introduce qemu_close_range()"
>> > - new patch: "util/async-teardown: Use qemu_close_range() to close fds"
>> > - Change to use qemu_close_range() to close fds for child process efficiently
>> > - v1 link: https://lore.kernel.org/qemu-devel/20230406112041.798585-1-bmeng@tinylab.org/
>> >
>> > Bin Meng (4):
>> >   tests/tcg/cris: Fix the coding style
>> >   tests/tcg/cris: Correct the off-by-one error
>> >   util/async-teardown: Fall back to close fds one by one
>> >   util/osdep: Introduce qemu_close_range()
>> >
>> > Zhangjin Wu (2):
>> >   util/async-teardown: Use qemu_close_range() to close fds
>> >   net: tap: Use qemu_close_range() to close fds
>> >
>>
>> Ping for 8.1?
>
> Queued.

There are review questions open on PATCH 4+5.