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

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Date:   Tue, 24 Aug 2021 19:54:54 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "rppt@...nel.org" <rppt@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "keescook@...omium.org" <keescook@...omium.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, 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.

> > > In my next series of this I added support for GFP_ATOMIC to this
> > > code,
> > > but that solution should only work for permission changing grouped
> > > page
> > > allocators in the protected page tables case where the direct map
> > > tables are handled differently. As a general solution though
> > > (that's
> > > the long term intention right?), GFP_ATOMIC might deserve some
> > > consideration.
> > 
> > ... but for the general solution GFP_ATOMIC indeed deserves some
> > consideration.
> >  
> > > The other thing is we probably don't want to clean out the atomic
> > > reserves and add them to a cache just for one page. I opted to just
> > > convert one page in the GFP_ATOMIC case.
> >  
> > Do you mean to allocate one page in GFP_ATOMIC case and bypass high
> > order
> > allocation?
> > But the CPA split is still necessary here, isn't it?
>
> Yes, grabs one atomic page and fragments it in the case of no pages in
> the grouped page cache. The CPA split is necessary still, but it should
> be ok because of the special way direct map page table allocations are
> handled for pks tables. Has not been reviewed by anyone yet, and
> wouldn't work as a general solution anyway.

-- 
Sincerely yours,
Mike.