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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6d354177d2fsm62931026d6.107.2024.11.05.10.46.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Nov 2024 10:46:42 -0800 (PST) Date: Tue, 5 Nov 2024 13:46:39 -0500 From: Peter Xu To: Ackerley Tng Cc: muchun.song@linux.dev, akpm@linux-foundation.org, rientjes@google.com, fvdl@google.com, jthoughton@google.com, david@redhat.com, isaku.yamahata@intel.com, zhiquan1.li@intel.com, fan.du@intel.com, jun.miao@intel.com, tabba@google.com, quic_eberman@quicinc.com, roypat@amazon.co.uk, jgg@nvidia.com, jhubbard@nvidia.com, seanjc@google.com, pbonzini@redhat.com, erdemaktas@google.com, vannapurve@google.com, pgonda@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH 2/3] mm: hugetlb: Refactor vma_has_reserves() to should_use_hstate_resv() Message-ID: References: <3d1946d01f63104de913c0979b5a596e2add1672.1728684491.git.ackerleytng@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <3d1946d01f63104de913c0979b5a596e2add1672.1728684491.git.ackerleytng@google.com> On Fri, Oct 11, 2024 at 11:22:37PM +0000, Ackerley Tng wrote: > With the addition of the chg parameter, vma_has_reserves() no longer > just determines whether the vma has reserves. > > The comment in the vma->vm_flags & VM_NORESERVE block indicates that > this function actually computes whether or not the reserved count > should be decremented. > > This refactoring also takes into account the allocation's request > parameter avoid_reserve, which helps to further simplify the calling > function alloc_hugetlb_folio(). > > Signed-off-by: Ackerley Tng I wonder if this patch could be merged with the 3rd, IIUC this can fundamentally be seen as a movement of what patch 3 was removed. Furthermore, I do feel like should_use_hstate_resv() could be redundant on its own on many things. Let me try to justify. Firstly, after 3 patches applied, now it looks like this (I removed all comments to make things shorter..): static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg, bool avoid_reserve) { if (avoid_reserve) return false; if (vma->vm_flags & VM_NORESERVE) { if (vma->vm_flags & VM_MAYSHARE && chg == 0) return true; else return false; } if (vma->vm_flags & VM_MAYSHARE) { if (chg) return false; else return true; } if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) { if (chg) return false; else return true; } return false; } Then let's look at chg==0 processing all above: what does it say? I suppose it means "we don't need another global reservation". It means if chg==0 we always will use an existing reservation. From math POV it also is true, so it can already be moved out ahead, IIUC, like this: static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg, bool avoid_reserve) { if (avoid_reserve) return false; if (chg == 0) return true; if (vma->vm_flags & VM_NORESERVE) return false; if (vma->vm_flags & VM_MAYSHARE) return false; if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) return false; return false; <--------------------- [1] } Move on. If I read it right, above [1] is exactly for avoid_reserve==1 case, where it basically says "it's !NORESERVE, private, and it's not the vma resv owner, either fork() or CoW". If my reading is correct, it means after your patch 2, [1] should never be reachable at all.. I would hope adding a panic() right above would be ok. And irrelevant of whether my understanding is correct.. math-wise above is also already the same as: static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg, bool avoid_reserve) { if (avoid_reserve) return false; if (chg == 0) return true; return false; } Then it makes a lot more sense now, because afaict, gbl_chg is exactly what should_use_hstate_resv() is looking for.. only except avoid_reserve==true. Would above make sense to you? In short, it's about whether a patch on top of your series would further simply this whole thing, like below: ===8<=== diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 60e72214d5bf..4b1c5c4ed7be 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -1245,80 +1245,6 @@ void clear_vma_resv_huge_pages(struct vm_area_struct *vma) hugetlb_dup_vma_private(vma); } -/* - * Returns true if this allocation should use (debit) hstate reservations, based on - * - * @vma: VMA config - * @chg: Whether the page requirement can be satisfied using subpool reservations - * @avoid_reserve: Whether allocation was requested to avoid using reservations - */ -static bool should_use_hstate_resv(struct vm_area_struct *vma, long chg, - bool avoid_reserve) -{ - if (avoid_reserve) - return false; - - if (vma->vm_flags & VM_NORESERVE) { - /* - * This address is already reserved by other process(chg == 0), - * so, we should decrement reserved count. Without decrementing, - * reserve count remains after releasing inode, because this - * allocated page will go into page cache and is regarded as - * coming from reserved pool in releasing step. Currently, we - * don't have any other solution to deal with this situation - * properly, so add work-around here. - */ - if (vma->vm_flags & VM_MAYSHARE && chg == 0) - return true; - else - return false; - } - - /* Shared mappings always use reserves */ - if (vma->vm_flags & VM_MAYSHARE) { - /* - * We know VM_NORESERVE is not set. Therefore, there SHOULD - * be a region map for all pages. The only situation where - * there is no region map is if a hole was punched via - * fallocate. In this case, there really are no reserves to - * use. This situation is indicated if chg != 0. - */ - if (chg) - return false; - else - return true; - } - - /* - * Only the process that called mmap() has reserves for private - * mappings. A child process with MAP_PRIVATE mappings created by their - * parent have no page reserves. - */ - if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) { - /* - * Like the shared case above, a hole punch or truncate - * could have been performed on the private mapping. - * Examine the value of chg to determine if reserves - * actually exist or were previously consumed. - * Very Subtle - The value of chg comes from a previous - * call to vma_needs_reserves(). The reserve map for - * private mappings has different (opposite) semantics - * than that of shared mappings. vma_needs_reserves() - * has already taken this difference in semantics into - * account. Therefore, the meaning of chg is the same - * as in the shared case above. Code could easily be - * combined, but keeping it separate draws attention to - * subtle differences. - */ - if (chg) - return false; - else - return true; - } - - return false; -} - static void enqueue_hugetlb_folio(struct hstate *h, struct folio *folio) { int nid = folio_nid(folio); @@ -3255,7 +3181,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma, } - use_hstate_resv = should_use_hstate_resv(vma, gbl_chg, avoid_reserve); + use_hstate_resv = avoid_reserve || !gbl_chg; /* * charge_cgroup_reservation if this allocation is not consuming a ===8<=== Thanks, -- Peter Xu