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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pybo1-003bwj-SW; Mon, 15 May 2023 17:16:17 +0000 Date: Mon, 15 May 2023 18:16:17 +0100 From: Matthew Wilcox To: Tarun Sahu Cc: linux-mm@kvack.org, akpm@linux-foundation.org, muchun.song@linux.dev, mike.kravetz@oracle.com, aneesh.kumar@linux.ibm.com, sidhartha.kumar@oracle.com, gerald.schaefer@linux.ibm.com, linux-kernel@vger.kernel.org, jaypatel@linux.ibm.com Subject: Re: [PATCH v2] mm/folio: Avoid special handling for order value 0 in folio_set_order Message-ID: References: <20230515170809.284680-1-tsahu@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230515170809.284680-1-tsahu@linux.ibm.com> X-Stat-Signature: c6c8kdes4df9fmc9qkw97recwuuoc3mo X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 76635C000B X-Rspam-User: X-HE-Tag: 1684170983-479090 X-HE-Meta: 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 nfXRwo06 sVCVqq7eg5ShOGTD5J9GJOMK9ZyryJjH2cQkBD+JiUfoBVP3U3+wUz2qZSkt5obNhRG302Jj+pfzSuC0q28Mw2P9um4x33/vpeCjr8NlCWJR9dtEZtoM09V9VUWXUX2ZGcTfGZFjS+WhOVHQtApOBQoDSlrun+Lbcky653vWhQHAmnH2HTulHWN56TQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.028726, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, May 15, 2023 at 10:38:09PM +0530, Tarun Sahu wrote: > @@ -1951,9 +1950,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio, > struct page *p; > > __folio_clear_reserved(folio); > - __folio_set_head(folio); > - /* we rely on prep_new_hugetlb_folio to set the destructor */ > - folio_set_order(folio, order); > for (i = 0; i < nr_pages; i++) { > p = folio_page(folio, i); > > @@ -1999,6 +1995,9 @@ static bool __prep_compound_gigantic_folio(struct folio *folio, > if (i != 0) > set_compound_head(p, &folio->page); > } > + __folio_set_head(folio); > + /* we rely on prep_new_hugetlb_folio to set the destructor */ > + folio_set_order(folio, order); This makes me nervous, as I said before. This means that compound_head(tail) can temporarily point to a page which is not marked as a head page. That's different from prep_compound_page(). You need to come up with some good argumentation for why this is safe, and no amount of testing you do can replace it -- any race in this area will be subtle.