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Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <20201028143442.GA20115@casper.infradead.org> References: <20201028143442.GA20115@casper.infradead.org> <160389418807.300137.8222864749005731859.stgit@warthog.procyon.org.uk> <160389426655.300137.17487677797144804730.stgit@warthog.procyon.org.uk> To: Matthew Wilcox Cc: dhowells@redhat.com, linux-afs@lists.infradead.org, kernel test robot , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 11/11] afs: Fix dirty-region encoding on ppc32 with 64K pages MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <541368.1603903981.1@warthog.procyon.org.uk> Content-Transfer-Encoding: quoted-printable Date: Wed, 28 Oct 2020 16:53:01 +0000 Message-ID: <541369.1603903981@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Matthew Wilcox wrote: > > +static inline unsigned int afs_page_dirty_resolution(void) > = > I've been using size_t for offsets within a struct page. I don't know > that we'll ever support pages larger than 2GB (they're completely > impractical with today's bus speeds), but I'd rather not be the one > who has to track down all the uses of 'int' in the kernel in fifteen > years time. Going beyond 2G page size won't be fun and a lot of our APIs will break - write_begin, write_end, invalidatepage to name a few. It would probably require an analysis program to trace all the usages of s= uch information within the kernel. > > +{ > > + if (PAGE_SIZE - 1 <=3D __AFS_PAGE_PRIV_MASK) > > + return 1; > > + else > > + return PAGE_SIZE / (__AFS_PAGE_PRIV_MASK + 1); > = > Could this be DIV_ROUND_UP(PAGE_SIZE, __AFS_PAGE_PRIV_MASK + 1); avoidin= g > a conditional? I appreciate it's calculated at compile time today, but > it'll be dynamic with THP. That seems to work. > > static inline unsigned int afs_page_dirty_to(unsigned long priv) > > { > > - return ((priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV_MASK) + 1; > > + unsigned int x =3D (priv >> __AFS_PAGE_PRIV_SHIFT) & __AFS_PAGE_PRIV= _MASK; > > + > > + /* The upper bound is exclusive */ > = > I think you mean 'inclusive'. The returned upper bound points immediately beyond the range. E.g. 0-0 is= an empty range. Changing that is way too big an overhaul outside the merge window. > > + return (x + 1) * afs_page_dirty_resolution(); > > } > > = > > static inline unsigned long afs_page_dirty(unsigned int from, unsigne= d int to) > > { > > + unsigned int res =3D afs_page_dirty_resolution(); > > + from /=3D res; /* Round down */ > > + to =3D (to + res - 1) / res; /* Round up */ > > return ((unsigned long)(to - 1) << __AFS_PAGE_PRIV_SHIFT) | from; > = > Wouldn't it produce the same result to just round down? ie: > = > to =3D (to - 1) / res; > return ((unsigned long)to << __AFS_PAGE_PRIV_SHIFT) | from; Actually, yes, because res/res=3D=3D1, which I then subtract afterwards. David