linux-kernel - Re: [PATCH] mm/page_alloc: fix the potential memory waste in page_frag_alloc

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Date:   Mon, 23 Oct 2023 17:17:41 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     wang wei <a929244872@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/page_alloc: fix the potential memory waste in
 page_frag_alloc_align

On Mon, Oct 23, 2023 at 11:42:16PM +0800, wang wei wrote:
> First step, allocating a memory fragment with size 1KB bytes uses
> page_frag_alloc_align.  It will allocate PAGE_FRAG_CACHE_MAX_SIZE
> bytes by __page_frag_cache_refill, store the pointer at nc->va and
> then return the last 1KB memory fragment which address is nc->va
> + PAGE_FRAG_CACHE_MAX_SIZE - 1KB. The remaining PAGE_FRAG_CACHE_MAX_SIZE
> - 1KB bytes of memory can Meet future memory requests.
> 
> Second step, if the caller requests a memory fragment with size
> more then PAGE_FRAG_CACHE_MAX_SIZE bytes,  page_frag_alloc_align,
> it will also allocate PAGE_FRAG_CACHE_MAX_SIZE bytes by
> __page_frag_cache_refill,  store the pointer at nc->va, and
> return NULL.  this behavior makes the rest of
> PAGE_FRAG_CACHE_MAX_SIZE - 1KB bytes memory at First step are
> wasted(allocate from buddy system but not used).

We could do this, but have you ever seen this happen, or are you
just reading code and looking for problems?  If the latter, I think
you've misunderstood how this allocator is normally used.