Re: [PATCH v1 06/36] mm/page_alloc: reject unreasonable folio/compound page sizes in alloc_contig_range_noprof()

[David Hildenbrand]

It seems a bit arbitrary, like we open-code this (at risk of making a mistake)
in some places but not others.

[...]


One could argue that maybe one would want a order_to_pages() helper (that
could use BIT() internally), but I am certainly not someone that would
suggest that at this point ...  :)

I mean maybe.

Anyway as I said none of this is massively important, the open-coding here is
correct, just seems silly.

Maybe we really want a ORDER_PAGES() and PAGES_ORDER().

But I mean, we also have PHYS_PFN() PFN_PHYS() and see how many "<< 
PAGE_SIZE" etc we are using all over the place.




+
   /*
    * compound_nr() returns the number of pages in this potentially compound
    * page.  compound_nr() can be called on a tail page, and is defined to
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index baead29b3e67b..426bc404b80cc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6833,6 +6833,7 @@ static int __alloc_contig_verify_gfp_mask(gfp_t gfp_mask, 
gfp_t *gfp_cc_mask)
   int alloc_contig_range_noprof(unsigned long start, unsigned long end,
                              acr_flags_t alloc_flags, gfp_t gfp_mask)

Funny btw th

   {
+       const unsigned int order = ilog2(end - start);
        unsigned long outer_start, outer_end;
        int ret = 0;

@@ -6850,6 +6851,9 @@ int alloc_contig_range_noprof(unsigned long start, 
unsigned long end,
                                            PB_ISOLATE_MODE_CMA_ALLOC :
                                            PB_ISOLATE_MODE_OTHER;

+       if (WARN_ON_ONCE((gfp_mask & __GFP_COMP) && order > MAX_FOLIO_ORDER))
+               return -EINVAL;

Possibly not worth it for a one off, but be nice to have this as a helper 
function, like:

static bool is_valid_order(gfp_t gfp_mask, unsigned int order)
{
        return !(gfp_mask & __GFP_COMP) || order <= MAX_FOLIO_ORDER;
}

Then makes this:

        if (WARN_ON_ONCE(!is_valid_order(gfp_mask, order)))
                return -EINVAL;

Kinda self-documenting!

I don't like it -- especially forwarding __GFP_COMP.

is_valid_folio_order() to wrap the order check? Also not sure.

OK, it's not a big deal.

Can we have a comment explaining this though? As people might be confused
as to why we check this here and not elsewhere.

I can add a comment.

--
Cheers

David / dhildenb