Re: [PATCH] bcachefs: Avoid a potential memory over-allocation in bch2_printbuf_make_room()

Brian Foster
2023-09-19T18:47:10Z

On Tue, Sep 19, 2023 at 08:34:00PM +0200, Christophe JAILLET wrote:
> Le 19/09/2023 à 15:18, Brian Foster a écrit :
> > On Sat, Sep 16, 2023 at 10:45:23AM +0200, Christophe JAILLET wrote:
> > > kmalloc() and co. don't always allocate a power of 2 number of bytes.
> > > There are some special handling for 64<n<=96 and 128<n<=192 cases.
> > >
> >
> > It's not immediately clear to me what you mean by "special handling."
> > Taking a quick look at slabinfo, it looks like what you mean is that
> > slab rounding is a bit more granular than power of two, particularly in
> > these ranges. Is that right? If so, JFYI it would be helpful to describe
> > that more explicitly in the commit log.
>
> That's what I tried to do with my 2 phrases.
> Sound good and clear to the French speaking man I am :)
>
> Would you mind updating the phrasing yourself?
> A trial and error method about wording with a non native English speaking
> person can be somewhat a long and boring experience to me.
>
> All what I could propose, with the help of google translate, is:
>
> "
> kmalloc() does not necessarily allocate a number of bytes equal to a power
> of two. There are special cases for sizes between 65 and 96 and between 129
> and 192. In these cases, 96 and 192 bytes are allocated respectively.
>
> So, instead of forcing an allocation always equal to a power of two, it may
> be interesting to use the same rounding rules as kmalloc(). This helps avoid
> over-allocating some memory.
>
> Use kmalloc_size_roundup() instead of roundup_pow_of_two().
> "
>
> If this is fine to you I can send a v2 with this wording, otherwise, either
> tweak it to what sounds good to you, or just ignore this patch.
>

I think that wording is fine. I don't think it's necessary to send a v2
just for a commit log update, but feel free to do so if you want.
Ultimately it will be up to Kent if he's alright with the change.

Brian

> CJ
>
> >
> > > So trust kmalloc() algorithm instead of forcing a power of 2 allocation.
> > > This can saves a few bytes of memory and still make use of all the
> > > memory allocated.
> > >
> > > On the other side, it may require an additional realloc() in some cases.
> > >
> >
> > Well, I feel like this isn't the only place I've seen the power of two
> > buffer size realloc algorithm thing, but in thinking about it this seems
> > fairly harmless and reasonable for printbufs. FWIW:
> >
> > Reviewed-by: Brian Foster <bfoster@redhat.com>
> >
> > > Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
> > > ---
> > > fs/bcachefs/printbuf.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/fs/bcachefs/printbuf.c b/fs/bcachefs/printbuf.c
> > > index 77bee9060bfe..34527407e950 100644
> > > --- a/fs/bcachefs/printbuf.c
> > > +++ b/fs/bcachefs/printbuf.c
> > > @@ -28,7 +28,7 @@ int bch2_printbuf_make_room(struct printbuf *out, unsigned extra)
> > > if (out->pos + extra < out->size)
> > > return 0;
> > > - new_size = roundup_pow_of_two(out->size + extra);
> > > + new_size = kmalloc_size_roundup(out->size + extra);
> > > /*
> > > * Note: output buffer must be freeable with kfree(), it's not required
> > > --
> > > 2.34.1
> > >
> >
> >
>