Re: [Lsf-pc] [LSF/MM/BPF TOPIC] Reclamation interactions with RCU

[Dan Carpenter <dan.carpenter@linaro.org>]

On Fri, Mar 22, 2024 at 12:47:42PM +1100, NeilBrown wrote:
> On Thu, 21 Mar 2024, Dan Carpenter wrote:
> > On Mon, Mar 04, 2024 at 09:45:48AM +1100, NeilBrown wrote:
> > > I have in mind a more explicit statement of how much waiting is
> > > acceptable.
> > > 
> > > GFP_NOFAIL - wait indefinitely
> > 
> > Why not call it GFP_SMALL?  It wouldn't fail.  The size would have to be
> > less than some limit.  If the size was too large, that would trigger a
> > WARN_ON_ONCE().
> 
> I would be happy with GFP_SMALL.  It would never return NULL but might
> block indefinitely.  It would (as you say) WARN (maybe ONCE) if the size
> was considered "COSTLY" and would possibly BUG if the size exceeded
> KMALLOC_MAX_SIZE. 

I'd like to keep GFP_SMALL much smaller than KMALLOC_MAX_SIZE.  IIf
you're allocating larger than that, you'd still be able to GFP_NOFAIL.
I looked quickly an I think over 60% of allocations are just sizeof(*p)
and probably 90% are under 4k.

> 
> > 
> > I obviously understand that this duplicates the information in the size
> > parameter but the point is that GFP_SMALL allocations have been
> > reviewed, updated, and don't have error handling code.
> 
> We are on the same page here.
> 
> > 
> > We'd keep GFP_KERNEL which would keep the existing behavior.  (Which is
> > that it can sleep and it can fail).  I think that maps to GFP_RETRY but
> > GFP_RETRY is an uglier name.
> 
> Can it fail though?  I know it is allowed to, but does it happen?
> 

In some sense, I don't really care about this, I just want the rules
clear from a static analysis perspective.  Btw, you're discussing making
the too small to fail rule official but other times we have discussed
getting rid of it all together.  So I think maybe it's better to keep
the rules strict but allow the actual implentation to change later.

The GFP_SMALL stuff is nice for static analysis because it would warn
about anything larger than whatever the small limit is.  So that means I
have fewer allocations to review for integer overflow bugs.

Btw, Jiri Pirko, was proposing a macro which would automatically
allocate the 60+% of allocations which are sizeof(*p).
https://lore.kernel.org/all/20240315132249.2515468-1-jiri@resnulli.us/
I had offered an alternative macro but the idea is the same:

#define __ALLOC(p) p __free(kfree) = kzalloc(sizeof(*p), GFP_KERNEL)
        struct ice_aqc_get_phy_caps_data *__ALLOC(pcaps);

Combining no fail allocations with automatic cleanup changes the way you
write code.

And then on the success patch you have the no_free_ptr() which I haven't
used but I think will be so useful for static analysis.  I'm so excited
about this.

regards,
dan carpenter