linux-kernel - Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped

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Message-ID: <CAPhsuW7_hb9T-uuB2FuXYMs7WA_XMbV809zOPSV7eOQeh4k3Qw@mail.gmail.com>
Date:   Fri, 19 May 2023 08:08:54 -0700
From:   Song Liu <song@...nel.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Mike Rapoport <rppt@...nel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH 1/5] mm: intorduce __GFP_UNMAPPED and unmapped_alloc()

On Thu, May 18, 2023 at 6:24 PM Kent Overstreet
<kent.overstreet@...ux.dev> wrote:
>
> On Thu, May 18, 2023 at 01:51:08PM -0700, Song Liu wrote:
>
> > To allocate memory for text, we will allocate 2MiB, make it ROX, and then
> > use it for many small allocations. IIUC, buddy allocator will use unallocated
> > parts of this page for metadata. I guess this may be a problem, as the
> > whole page is ROX now, and we have to use text_poke to write to it.
>
> The standard kernel buddy allocator does _not_ store anything in the
> page itself - because the page might be a highmem page.

Thanks for the correction.
Song
>
> That's also why I went with the bitmap for my slab allocator; standard
> kernel slab allocator stores a freelist ptr in free objects.