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Fri, 6 Jun 2025 08:20:14 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id OmoQO76kQmgbBAAAD6G6ig (envelope-from ); Fri, 06 Jun 2025 08:20:14 +0000 Date: Fri, 6 Jun 2025 10:20:13 +0200 From: Oscar Salvador To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, Andrew Morton , Alistair Popple , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Zi Yan , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Dan Williams Subject: Re: [PATCH v1 1/2] mm/huge_memory: don't mark refcounted pages special in vmf_insert_folio_pmd() Message-ID: References: <20250603211634.2925015-1-david@redhat.com> <20250603211634.2925015-2-david@redhat.com> Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250603211634.2925015-2-david@redhat.com> X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_TWELVE(0.00)[19]; ASN(0.00)[asn:25478, ipnet:::/0, country:RU]; MISSING_XM_UA(0.00)[]; RCVD_TLS_ALL(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim,localhost.localdomain:mid,imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] X-Spam-Level: X-Spam-Flag: NO X-Rspamd-Queue-Id: 12EE91F46E X-Rspamd-Action: no action X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Spam-Score: -4.51 On Tue, Jun 03, 2025 at 11:16:33PM +0200, David Hildenbrand wrote: > Marking PMDs that map a "normal" refcounted folios as special is > against our rules documented for vm_normal_page(). > > Fortunately, there are not that many pmd_special() check that can be > mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that > would get this wrong right now are rather harmless: e.g., none so far > bases decisions whether to grab a folio reference on that decision. > > Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big > implications as it seems. > > Getting this right will get more important as we use > folio_normal_page_pmd() in more places. > > Fix it by just inlining the relevant code, making the whole > pmd_none() handling cleaner. We can now use folio_mk_pmd(). > > While at it, make sure that a pmd that is not-none is actually present > before comparing PFNs. > > Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()") > Signed-off-by: David Hildenbrand Hi David, > --- > mm/huge_memory.c | 39 ++++++++++++++++++++++++++++++++------- > 1 file changed, 32 insertions(+), 7 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index d3e66136e41a3..f9e23dfea76f8 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1474,9 +1474,10 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio, > struct vm_area_struct *vma = vmf->vma; > unsigned long addr = vmf->address & PMD_MASK; > struct mm_struct *mm = vma->vm_mm; > + pmd_t *pmd = vmf->pmd; > spinlock_t *ptl; > pgtable_t pgtable = NULL; > - int error; > + pmd_t entry; > > if (addr < vma->vm_start || addr >= vma->vm_end) > return VM_FAULT_SIGBUS; > @@ -1490,17 +1491,41 @@ vm_fault_t vmf_insert_folio_pmd(struct vm_fault *vmf, struct folio *folio, > return VM_FAULT_OOM; > } > > - ptl = pmd_lock(mm, vmf->pmd); > - if (pmd_none(*vmf->pmd)) { > + ptl = pmd_lock(mm, pmd); > + if (pmd_none(*pmd)) { > folio_get(folio); > folio_add_file_rmap_pmd(folio, &folio->page, vma); > add_mm_counter(mm, mm_counter_file(folio), HPAGE_PMD_NR); > + > + entry = folio_mk_pmd(folio, vma->vm_page_prot); > + if (write) { > + entry = pmd_mkyoung(pmd_mkdirty(entry)); > + entry = maybe_pmd_mkwrite(entry, vma); > + } > + set_pmd_at(mm, addr, pmd, entry); > + update_mmu_cache_pmd(vma, addr, pmd); > + > + if (pgtable) { > + pgtable_trans_huge_deposit(mm, pmd, pgtable); > + mm_inc_nr_ptes(mm); > + pgtable = NULL; > + } > + } else if (pmd_present(*pmd) && write) { > + /* > + * We only allow for upgrading write permissions if the > + * same folio is already mapped. > + */ > + if (pmd_pfn(*pmd) == folio_pfn(folio)) { > + entry = pmd_mkyoung(*pmd); > + entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma); > + if (pmdp_set_access_flags(vma, addr, pmd, entry, 1)) > + update_mmu_cache_pmd(vma, addr, pmd); > + } else { > + WARN_ON_ONCE(!is_huge_zero_pmd(*pmd)); > + } So, this is pretty much insert_pfn_pmd without pmd_mkdevmap/pmd_mkspecial(). I guess vmf_inser_folio_pmd() doesn't have to be concerned with devmaps either, right? Looks good to me, just a nit: would it not be better to pass a boolean to insert_pfn_pmd() that lets it know whether it "can" create a devmap/special entries? -- Oscar Salvador SUSE Labs