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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pHUjd-008zPN-Oh; Mon, 16 Jan 2023 19:01:33 +0000 Date: Mon, 16 Jan 2023 19:01:33 +0000 From: Matthew Wilcox To: Baoquan He Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, urezki@gmail.com, lstoakes@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: <20230113031921.64716-4-bhe@redhat.com> X-Rspamd-Queue-Id: 0D05516001B X-Stat-Signature: 3zzehmip8ih64rmkuikgruh51w8otjie X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1673895686-888312 X-HE-Meta: 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 ...