Re: [libvirt RFC] qemu_saveimage: only use wrapper when using --bypass-cache

On Wed, Apr 13, 2022 at 12:49 AM Claudio Fontana <cfontana(a)suse.de&gt; wrote: > > On 4/12/22 7:23 PM, Ani Sinha wrote: >> On Tue, Apr 12, 2022 at 10:09 PM Ani Sinha <ani(a)anisinha.ca&gt; wrote: >> >>> On Tue, Apr 12, 2022 at 2:48 PM Claudio Fontana <cfontana(a)suse.de&gt; wrote: >>>> >>>> align the "save" with the "restore" code, >>>> by only using the wrapper when using --bypass-cache. >>>> >>>> This avoids a copy, resulting in better performance. >>>> >>>> Signed-off-by: Claudio Fontana <cfontana(a)suse.de&gt; >>>> --- >>>> src/qemu/qemu_saveimage.c | 6 ++++-- >>>> 1 file changed, 4 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/src/qemu/qemu_saveimage.c b/src/qemu/qemu_saveimage.c >>>> index 4fd4c5cfcd..5ea1b2fbcc 100644 >>>> --- a/src/qemu/qemu_saveimage.c >>>> +++ b/src/qemu/qemu_saveimage.c >>>> @@ -289,8 +289,10 @@ qemuSaveImageCreate(virQEMUDriver *driver, >>>> if (qemuSecuritySetImageFDLabel(driver->securityManager, vm->def, >>> fd) < 0) >>>> goto cleanup; >>>> >>>> - if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) >>>> - goto cleanup; >>>> + if ((flags & VIR_DOMAIN_SAVE_BYPASS_CACHE)) { >>>> + if (!(wrapperFd = virFileWrapperFdNew(&fd, path, wrapperFlags))) >>>> + goto cleanup; >>>> + } >>> >>> There is an obvious issue with this code. We are trying to close and >>> free a file descriptor that we have not opened when >>> VIR_DOMAIN_SAVE_BYPASS_CACHE is set in flags. >> >> >> I meant *not* set in flags. > > I don't think so. I don't think it is obvious, so can you be more specific? See if (qemuDomainFileWrapperFDClose(vm, wrapperFd) < 0) goto cleanup; and under cleanup: if (qemuDomainFileWrapperFDClose(vm, wrapperFd) < 0) ret = -1; if VIR_DOMAIN_SAVE_BYPASS_CACHE not set in flags, both the above function calls use wrapperFd initialized to NULL. I think this patch would be incomplete if you do not handle all these scenarios. > > From the function header comments it seems lecit and defined to call the function with NULL: > > /** > * virFileWrapperFdClose: > * @wfd: fd wrapper, or NULL > * > * If @wfd is valid, then ensure that I/O has completed, which may > * include reaping a child process. Return 0 if all data for the > * wrapped fd is complete, or -1 on failure with an error emitted. > * This function intentionally returns 0 when @wfd is NULL, so that > * callers can conditionally create a virFileWrapperFd wrapper but > * unconditionally call the cleanup code. To avoid deadlock, only > * call this after closing the fd resulting from virFileWrapperFdNew(). > * > * This function can be safely called multiple times on the same @wfd. > */ > > Seems it has been specifically designed to simplify situations like this. >