Re: [PATCH v3 01/22] mm: introduce xvmalloc() et al and use for grant table allocations

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, May 03, 2021 at 03:50:48PM +0200, Jan Beulich wrote:
> On 03.05.2021 13:31, Roger Pau Monné wrote:
> > On Thu, Apr 22, 2021 at 04:43:39PM +0200, Jan Beulich wrote:
> >> All of the array allocations in grant_table_init() can exceed a page's
> >> worth of memory, which xmalloc()-based interfaces aren't really suitable
> >> for after boot. We also don't need any of these allocations to be
> >> physically contiguous.. Introduce interfaces dynamically switching
> >> between xmalloc() et al and vmalloc() et al, based on requested size,
> >> and use them instead.
> >>
> >> All the wrappers in the new header get cloned mostly verbatim from
> >> xmalloc.h, with the sole adjustment to switch unsigned long to size_t
> >> for sizes and to unsigned int for alignments.
> > 
> > We seem to be growing a non-trivial amount of memory allocation
> > families of functions: xmalloc, vmalloc and now xvmalloc.
> > 
> > I think from a consumer PoV it would make sense to only have two of
> > those: one for allocations that require to be physically contiguous,
> > and one for allocation that don't require it.
> > 
> > Even then, requesting for physically contiguous allocations could be
> > done by passing a flag to the same interface that's used for
> > non-contiguous allocations.
> > 
> > Maybe another option would be to expand the existing
> > v{malloc,realloc,...} set of functions to have your proposed behaviour
> > for xv{malloc,realloc,...}?
> 
> All of this and some of your remarks further down has already been
> discussed. A working group has been formed. No progress since. Yes,
> a smaller set of interfaces may be the way to go. Controlling
> behavior via flags, otoh, is very much not malloc()-like. Making
> existing functions have the intended new behavior is a no-go without
> auditing all present uses, to find those few which actually may need
> physically contiguous allocations.

But you could make your proposed xvmalloc logic the implementation
behind vmalloc, as that would still be perfectly fine and safe? (ie:
existing users of vmalloc already expect non-physically contiguous
memory). You would just optimize the size < PAGE_SIZE for that
interface?

> Having seen similar naming elsewhere, I did propose xnew() /
> xdelete() (plus array and flex-struct counterparts) as the single
> new recommended interface; didn't hear back yet. But we'd switch to
> that gradually, so intermediately there would still be a larger set
> of interfaces.
> 
> I'm not convinced we should continue to have byte-granular allocation
> functions producing physically contiguous memory. I think the page
> allocator should be used directly in such cases.
> 
> >> --- /dev/null
> >> +++ b/xen/include/xen/xvmalloc.h
> >> @@ -0,0 +1,73 @@
> >> +
> >> +#ifndef __XVMALLOC_H__
> >> +#define __XVMALLOC_H__
> >> +
> >> +#include <xen/cache.h>
> >> +#include <xen/types.h>
> >> +
> >> +/*
> >> + * Xen malloc/free-style interface for allocations possibly exceeding a page's
> >> + * worth of memory, as long as there's no need to have physically contiguous
> >> + * memory allocated.  These should be used in preference to xmalloc() et al
> >> + * whenever the size is not known to be constrained to at most a single page.
> > 
> > Even when it's know that size <= PAGE_SIZE this helpers are
> > appropriate as they would end up using xmalloc, so I think it's fine to
> > recommend them universally as long as there's no need to alloc
> > physically contiguous memory?
> > 
> > Granted there's a bit more overhead from the logic to decide between
> > using xmalloc or vmalloc &c, but that's IMO not that big of a deal in
> > order to not recommend this interface globally for non-contiguous
> > alloc.
> 
> As long as xmalloc() and vmalloc() are meant stay around as separate
> interfaces, I wouldn't want to "forbid" their use when it's sufficiently
> clear that they would be chosen by the new function anyway. Otoh, if the
> new function became more powerful in terms of falling back to the

What do you mean with more powerful here?

Thanks, Roger.