linux-kernel - Re: [PATCH v2 07/12] s390: add pte_free

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Message-ID: <20230629175645.7654d0a8@thinkpad-T15>
Date:   Thu, 29 Jun 2023 17:56:45 +0200
From:   Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Huang Ying <ying.huang@...el.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Jann Horn <jannh@...gle.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables
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On Thu, 29 Jun 2023 12:22:24 -0300
Jason Gunthorpe <jgg@...pe.ca> wrote:

> On Wed, Jun 28, 2023 at 10:08:08PM -0700, Hugh Dickins wrote:
> > On Wed, 28 Jun 2023, Gerald Schaefer wrote:  
> > > 
> > > As discussed in the other thread, we would rather go with less complexity,
> > > possibly switching to an approach w/o the list and fragment re-use in the
> > > future. For now, as a first step in that direction, we can try with not
> > > adding fragments back only for pte_free_defer(). Here is an adjusted
> > > version of your patch, copying most of your pte_free_defer() logic and
> > > also description, tested with LTP and all three of your patch series applied:  
> > 
> > Thanks, Gerald: I don't mind abandoning my 13/12 SLAB_TYPESAFE_BY_RCU
> > patch (posted with fewer Cc's to the s390 list last week), and switching
> > to your simpler who-cares-if-we-sometimes-don't-make-maximal-use-of-page
> > patch.
> > 
> > But I didn't get deep enough into it today to confirm it - and disappointed
> > that you've found it necessary to play with pt_frag_refcount in addition to
> > _refcount and HH bits.  No real problem with that, but my instinct says it
> > should be simpler.  

Yes, I also found it a bit awkward, but it seemed "good and simple enough",
to have something to go forward with, while my instinct was in line with yours.

> 
> Is there any reason it should be any different at all from what PPC is
> doing?
> 
> I still think the right thing to do here is make the PPC code common
> (with Hugh's proposed RCU modification) and just use it in both
> arches....

With the current approach, we would not add back fragments _only_ for
the new pte_free_defer() path, while keeping our cleverness for the other
paths. Not having a good overview of the negative impact wrt potential
memory waste, I would rather take small steps, if possible.

If we later switch to never adding back fragments, of course we should
try to be in line with PPC implementation.