On 3/4/23, Al Viro [off-list ref] wrote:

On Fri, Mar 03, 2023 at 09:39:11PM +0100, Mateusz Guzik wrote:

quoted

the allocation routine does not have any information about the size
available at compilation time, so has to resort to a memset call at
runtime. Instead, should this be:

f = kmem_cache_alloc(...);
memset(f, 0, sizeof(*f));

... the compiler could in principle inititalize stuff as indicated by
code and emit zerofill for the rest. Interestingly, last I checked
neither clang nor gcc knew how to do it, they instead resort to a full
sized memset anyway, which is quite a bummer.

For struct file I wouldn't expect a win from that, TBH.

That was mostly for illustrative purposes, but you are right -- turns
out the slab is 256 bytes in size per obj and only a small fraction of
it is inititalized in the allocation routine. Good candidate to always
punt to memset in the allocator as it happens now.

Bummer though, it is also one of the 2 most memset'ed during kernel
build. The other one allocated at the same rate is lsm_file_cache and
that's only 32 bytes in size, so it will get a win.

-- 
Mateusz Guzik <mjguzik gmail.com>