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Thu, 04 Apr 2024 12:41:09 -0700 (PDT) MIME-Version: 1.0 References: <20240404162515.527802-1-fvdl@google.com> <20240404162515.527802-2-fvdl@google.com> In-Reply-To: From: Frank van der Linden Date: Thu, 4 Apr 2024 12:40:58 -0700 Message-ID: Subject: Re: [PATCH 2/2] mm/hugetlb: pass correct order_per_bit to cma_declare_contiguous_nid To: Roman Gushchin Cc: linux-mm@kvack.org, muchun.song@linux.dev, akpm@linux-foundation.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 6B75C20017 X-Stat-Signature: 88w4t3o3sjswww6k1ekymf4bnjsxuf9n X-HE-Tag: 1712259670-803776 X-HE-Meta: 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 OTz2qQsT XZec8t9CWxJrpHUMQeeslVEdalbDnwjyWjQZMEDjeKJf2s7QJEurKitft1DCTAV5JwAiJGC64qvp3hH88ToS2gCw9sLIYXWtvkCBtoUma5MLFRXEfgBQ+oNswLD0Fpr/+CrfhydoStHOGlkBgZyMmDcKUYsQOeLepq3z6R1UQnWr4mRFq703dwaTjPB7HFq0O0jFDlmkhBZN5TD+Z7TAsL3EpknqFgH00Okjc/gYZXGA3dRJ1cSlKseT3+DXwF4aDgjPbV1LSS7jtmgnTPCy73Ol64QFFD2xEEAudHlkpoIIoxKE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000022, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Apr 4, 2024 at 11:56=E2=80=AFAM Roman Gushchin wrote: > > On Thu, Apr 04, 2024 at 04:25:15PM +0000, Frank van der Linden wrote: > > The hugetlb_cma code passes 0 in the order_per_bit argument to > > cma_declare_contiguous_nid (the alignment, computed using the > > page order, is correctly passed in). > > > > This causes a bit in the cma allocation bitmap to always represent > > a 4k page, making the bitmaps potentially very large, and slower. > > > > So, correctly pass in the order instead. > > > > Signed-off-by: Frank van der Linden > > Cc: Roman Gushchin > > Fixes: cf11e85fc08c ("mm: hugetlb: optionally allocate gigantic hugepag= es using cma") > > Hi Frank, > > there is a comment just above your changes which explains why order_per_b= it is 0. > Is this not true anymore? If so, please, fix the comment too. Please, cla= rify. > > Thanks! Hi Roman, I'm assuming you're referring to this comment: /* * Note that 'order per bit' is based on smallest size that * may be returned to CMA allocator in the case of * huge page demotion. */ That comment was added in a01f43901cfb9 ("hugetlb: be sure to free demoted CMA pages to CMA"). It talks about HUGETLB_PAGE_ORDER being the minimum order being given back to the CMA allocator (after hugetlb demotion), therefore order_per_bit must be HUGETLB_PAGE_ORDER. See the commit message for that commit: "Therefore, at region setup time we use HUGETLB_PAGE_ORDER as the smallest possible huge page size that can be given back to CMA." But the commit, while correctly changing the alignment, left the order_per_bit argument at 0, even though it clearly intended to set it at HUGETLB_PAGE_ORDER. The confusion may have been that cma_declare_contiguous_nid has 9 arguments, several of which can be left at 0 meaning 'use default', so it's easy to misread. In other words, the comment was correct, but the code was not. After this patch, comment and code match.