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Mon, 19 Dec 2022 05:01:44 -0800 (PST) Date: Mon, 19 Dec 2022 13:01:43 +0000 From: Lorenzo Stoakes To: Baoquan He Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, urezki@gmail.com, stephen.s.brennan@oracle.com, willy@infradead.org, akpm@linux-foundation.org, hch@infradead.org Subject: Re: [PATCH v2 2/7] mm/vmalloc.c: add flags to mark vm_map_ram area Message-ID: References: <20221217015435.73889-1-bhe@redhat.com> <20221217015435.73889-3-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-Queue-Id: 519BD1A0030 X-Rspamd-Server: rspam01 X-Stat-Signature: wyfd987ab4rx5fzmxx3m9dpdu4tph7n9 X-HE-Tag: 1671454906-170107 X-HE-Meta: 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 rkz6vu7T kRtqcSMJ0UyvbR5hajgHn1f1Y/f5e0+MZP1VliyKOA5zUHdw6B15FKl8JWaholUQELtHQOpR9w2tp4FGFwApqIKmQReUAistMdUrNoh/bVNVagWNSZwQfYifLKkQi/UR2aR+i 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 Mon, Dec 19, 2022 at 08:24:47PM +0800, Baoquan He wrote: > In fact, I should not do the checking, but do the clearing anyway. If I > change it as below, does it look better to you? > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index 5e578563784a..369b848d097a 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -2253,8 +2253,7 @@ void vm_unmap_ram(const void *mem, unsigned int count) > spin_lock(&vmap_area_lock); > va = __find_vmap_area((unsigned long)addr, &vmap_area_root); > BUG_ON(!va); > - if (va) > - va->flags &= ~VMAP_RAM; > + va->flags &= ~VMAP_RAM; > spin_unlock(&vmap_area_lock); > debug_check_no_locks_freed((void *)va->va_start, > (va->va_end - va->va_start)); This is better as it avoids the slightly contradictory situation of checking for a condition we've asserted is not the case, but I would still far prefer keeping this as-is and placing the unlock before the BUG_ON(). This avoids explicitly and knowingly holding a lock over a BUG_ON() and is simple to implement, e.g.:- spin_lock(&vmap_area_lock); va = __find_vmap_area((unsigned long)addr, &vmap_area_root); if (va) va->flags &= ~VMAP_RAM; spin_unlock(&vmap_area_lock); BUG_ON(!va); > > You are at this point clearing the VMAP_RAM flag though, so if it is unimportant > > what the flags are after this call, why are you clearing this one? > > With my understanding, We had better do the clearing. Currently, from > the code, not doing the clearing won't cause issue. If possible, I would > like to clear it as below draft code. > Sure, it seems appropriate to clear it, I'm just unsure as to why you aren't just clearing both flags? Perhaps just set va->flags = 0? > > > > It is just a little confusing, I wonder whether the VMAP_BLOCK flag is necessary > > at all, is it possible to just treat a non-VMAP_BLOCK VMAP_RAM area as if it > > were simply a fully occupied block? Do we gain much by the distinction? > > Yeah, VMAP_BLOCK flag is necessary. vmap_block contains used region, > or dirty/free regions. While the non-vmap_blcok vm_map_ram area is > similar with the non vm_map_ram area. When reading out vm_map_ram > regions, vmap_block regions need be treated differently. OK looking through again closely I see you're absolutely right, I wondered whether you could somehow make a non-VMAP_BLOCK vread() operation be equivalent to a block one (only across the whole mapping), but I don't think you can.