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[86.164.169.89]) by smtp.gmail.com with ESMTPSA id j20-20020a05600c1c1400b003daf89e01d3sm5910223wms.11.2023.01.16.11.48.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Jan 2023 11:48:13 -0800 (PST) Date: Mon, 16 Jan 2023 19:48:12 +0000 From: Lorenzo Stoakes To: Matthew Wilcox Cc: Baoquan He , linux-mm@kvack.org, linux-kernel@vger.kernel.org, urezki@gmail.com, stephen.s.brennan@oracle.com, akpm@linux-foundation.org, hch@infradead.org Subject: Re: [PATCH v3 3/7] mm/vmalloc.c: allow vread() to read out vm_map_ram areas Message-ID: References: <20230113031921.64716-1-bhe@redhat.com> <20230113031921.64716-4-bhe@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: a1dpieg4khxogkyempkyacrto6xyzpwu X-Rspamd-Queue-Id: 06523100014 X-HE-Tag: 1673898495-472010 X-HE-Meta: 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 ALM1bbcB GdCunerjz3Z0GJJxqWucWQtfFFIrKfopDsZsP734kW0elA4oI+hdZjuW5U4y6zZSQ8o4cR0IAIA1Ec27LU74payLNLNCz2KU1CSxqIShKw/yHXV1q+vPw3DA/qJqrll4sxdcV X-Bogosity: Ham, tests=bogofilter, spamicity=0.000076, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Jan 16, 2023 at 07:01:33PM +0000, Matthew Wilcox wrote: > On Fri, Jan 13, 2023 at 11:19:17AM +0800, Baoquan He wrote: > > + spin_lock(&vb->lock); > > + if (bitmap_empty(vb->used_map, VMAP_BBMAP_BITS)) { > > + spin_unlock(&vb->lock); > > + memset(buf, 0, count); > > + return; > > + } > > + for_each_set_bitrange(rs, re, vb->used_map, VMAP_BBMAP_BITS) { > > + if (!count) > > + break; > > + start = vmap_block_vaddr(vb->va->va_start, rs); > > + while (addr < start) { > > + if (count == 0) > > + break; > > + *buf = '\0'; > > + buf++; > > + addr++; > > + count--; > > + } > > + /*it could start reading from the middle of used region*/ > > + offset = offset_in_page(addr); > > + n = ((re - rs + 1) << PAGE_SHIFT) - offset; > > + if (n > count) > > + n = count; > > + aligned_vread(buf, start+offset, n); > > The whole vread() interface is rather suboptimal. The only user is proc, > which is trying to copy to userspace. But the vread() interface copies > to a kernel address, so kcore has to copy to a bounce buffer. That makes > this spinlock work, but the price is that we can't copy to a user address > in the future. Ideally, read_kcore() would be kcore_read_iter() and > we'd pass an iov_iter into vread(). vread() would then need to use a > mutex rather than a spinlock. > > I don't think this needs to be done now, but if someone's looking for > a project ... Interesting! I may take a look at this if I get the time :)