linux-kernel - Re: [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE

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Date:   Tue, 24 Aug 2021 17:23:04 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "Lutomirski, Andy" <luto@...nel.org>
Subject: Re: [RFC PATCH 3/4] mm/page_alloc: introduce __GFP_PTE_MAPPED flag to
 allocate pte-mapped pages

On Tue, 2021-08-24 at 19:54 +0300, Mike Rapoport wrote:
> On Tue, Aug 24, 2021 at 04:38:03PM +0000, Edgecombe, Rick P wrote:
> > On Tue, 2021-08-24 at 16:02 +0300, Mike Rapoport wrote:
> > > > We probably want to exclude GFP_ATOMIC before calling into CPA
> > > > unless
> > > > debug page alloc is on, because it may need to split and sleep
> > > > for
> > > > the
> > > > allocation. There is a page table allocation with GFP_ATOMIC
> > > > passed
> > > > actually.
> > > 
> > > Looking at the callers of alloc_low_pages() it seems that
> > > GFP_ATOMIC
> > > there
> > > is stale...
> > 
> > Well two actually, there is also spp_getpage(). I tried to
> > determine if
> > that was also stale but wasn't confident. There were a lot of paths
> > in.
> 
>   
> It's also used at init and during memory hotplug, so I really doubt
> it
> needs GFP_ATOMIC.

Pretty sure it gets called after init by at least something besides
hotplug. I saw it during debugging with a little sanitizer I built to
find any unprotected page tables missed. Something tweaking the fixmap
IIRC. Did you look at the set_fixmap_() and set_pte_vaddr() family of
functions? Now whether any of them actually need GFP_ATOMIC, I am less
sure. There were a fair amount of drivers to analyze.