linux-kernel - Re: [PATCH v2] mm/sparse.c: Use kvmalloc_node/kvfree to alloc/free memmap for the classic sparse

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Message-ID: <20200313000041.GM22433@bombadil.infradead.org>
Date:   Thu, 12 Mar 2020 17:00:41 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     Baoquan He <bhe@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mhocko@...e.com, akpm@...ux-foundation.org,
        david@...hat.com
Subject: Re: [PATCH v2] mm/sparse.c: Use kvmalloc_node/kvfree to alloc/free
 memmap for the classic sparse

On Thu, Mar 12, 2020 at 10:50:55PM +0000, Wei Yang wrote:
> On Thu, Mar 12, 2020 at 07:25:35AM -0700, Matthew Wilcox wrote:
> >Yes, I thought about that.  I decided it wasn't a problem, as long as
> >the struct page remains aligned, and we now have a guarantee that allocations
> >above 512 bytes in size are aligned.  With a 64 byte struct page, as long
> 
> Where is this 512 bytes condition comes from?

Filesystems need to do I/Os from kmalloc addresses and those I/Os need to
be 512 byte aligned.

> >as we're allocating at least 8 pages, we know it'll be naturally aligned.
> >
> >Your calculation doesn't take into account the size of struct page.
> >128M / 64k is indeed 2k, but you forgot to multiply by 64, which takes
> >us to 128kB.
> 
> You are right. While would there be other combination? Or in the future?
> 
> For example, there are definitions of
> 
> #define SECTION_SIZE_BITS       26
> #define SECTION_SIZE_BITS       24
> 
> Are we sure it won't break some thing?

As I said, once it's at least 512 bytes, it'll be 512 byte aligned.  And I
can't see us having sections smaller than 8 pages, can you?