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([2a02:6b6f:e750:1b00:1cfc:9209:4810:3ae5]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fb871d051sm7634404f8f.14.2025.09.26.08.27.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 26 Sep 2025 08:27:26 -0700 (PDT) Message-ID: <035a8839-c786-45b6-8458-87ac1c48f3bc@gmail.com> Date: Fri, 26 Sep 2025 16:27:25 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 mm-new 06/12] mm: thp: enable THP allocation exclusively through khugepaged Content-Language: en-GB To: Yafang Shao , akpm@linux-foundation.org, david@redhat.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, hannes@cmpxchg.org, gutierrez.asier@huawei-partners.com, willy@infradead.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, ameryhung@gmail.com, rientjes@google.com, corbet@lwn.net, 21cnbao@gmail.com, shakeel.butt@linux.dev, tj@kernel.org, lance.yang@linux.dev Cc: bpf@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org References: <20250926093343.1000-1-laoar.shao@gmail.com> <20250926093343.1000-7-laoar.shao@gmail.com> From: Usama Arif In-Reply-To: <20250926093343.1000-7-laoar.shao@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 26/09/2025 10:33, Yafang Shao wrote: > khugepaged_enter_vma() ultimately invokes any attached BPF function with > the TVA_KHUGEPAGED flag set when determining whether or not to enable > khugepaged THP for a freshly faulted in VMA. > > Currently, on fault, we invoke this in do_huge_pmd_anonymous_page(), as > invoked by create_huge_pmd() and only when we have already checked to > see if an allowable TVA_PAGEFAULT order is specified. > > Since we might want to disallow THP on fault-in but allow it via > khugepaged, we move things around so we always attempt to enter > khugepaged upon fault. > > This change is safe because: > - the checks for thp_vma_allowable_order(TVA_KHUGEPAGED) and > thp_vma_allowable_order(TVA_PAGEFAULT) are functionally equivalent hmm I dont think this is the case. __thp_vma_allowable_orders deals with TVA_PAGEFAULT (in_pf) differently from TVA_KHUGEPAGED. > - khugepaged operates at the MM level rather than per-VMA. The THP > allocation might fail during page faults due to transient conditions > (e.g., memory pressure), it is safe to add this MM to khugepaged for > subsequent defragmentation. > > While we could also extend prctl() to utilize this new policy, such a > change would require a uAPI modification to PR_SET_THP_DISABLE. > > Signed-off-by: Yafang Shao > Acked-by: Lance Yang > --- > mm/huge_memory.c | 1 - > mm/memory.c | 13 ++++++++----- > 2 files changed, 8 insertions(+), 6 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 08372dfcb41a..2b155a734c78 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1346,7 +1346,6 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf) > ret = vmf_anon_prepare(vmf); > if (ret) > return ret; > - khugepaged_enter_vma(vma); > > if (!(vmf->flags & FAULT_FLAG_WRITE) && > !mm_forbids_zeropage(vma->vm_mm) && > diff --git a/mm/memory.c b/mm/memory.c > index 58ea0f93f79e..64f91191ffff 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -6327,11 +6327,14 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma, > if (pud_trans_unstable(vmf.pud)) > goto retry_pud; > > - if (pmd_none(*vmf.pmd) && > - thp_vma_allowable_order(vma, TVA_PAGEFAULT, PMD_ORDER)) { > - ret = create_huge_pmd(&vmf); > - if (!(ret & VM_FAULT_FALLBACK)) > - return ret; > + if (pmd_none(*vmf.pmd)) { > + if (vma_is_anonymous(vma)) > + khugepaged_enter_vma(vma); > + if (thp_vma_allowable_order(vma, TVA_PAGEFAULT, PMD_ORDER)) { > + ret = create_huge_pmd(&vmf); > + if (!(ret & VM_FAULT_FALLBACK)) > + return ret; > + } > } else { > vmf.orig_pmd = pmdp_get_lockless(vmf.pmd); >