From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id C921467729; Tue, 30 Jan 2024 11:33:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706614404; cv=none; b=vBzUC+BEpEn4n84TB1XHk6ndbtuKx/8SUpu0Y01pRWorOAz2MHe/TaYGCHCcw8zlqI7uCvheXTfC2ba0zBXQTl7/rcQaSo6afq1KPpE/PfJ4fluH/nWc4SBj+Coz+4JZJ2RRJZFx0CS/RVM64Y8qgnlQnEvR371rAH9vTKcw4E4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706614404; c=relaxed/simple; bh=iYeZRBQMYetEM+TK5yHlgX6GcwqR9jGOdY9NNlijHeI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JnNG0P5FvhwG3Cd6UOqZaR8H4RrXt5Ai5rtYR5pcSG1+OUDNtl8fG1Sudeydh9SonyCekVNIRJS0kshi/87l9HhXQWOxWAioaM63XfDFHOs6gGscT2BLoqO2eg62Gv8jgQjLdFf6wwaVijkgzkpI/vUeWWztgrI+psLwdgIjdTA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 121DBDA7; Tue, 30 Jan 2024 03:34:03 -0800 (PST) Received: from raptor (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EA3D43F5A1; Tue, 30 Jan 2024 03:33:13 -0800 (PST) Date: Tue, 30 Jan 2024 11:33:07 +0000 From: Alexandru Elisei To: Anshuman Khandual Cc: catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev, maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com, yuzenghui@huawei.com, arnd@arndb.de, akpm@linux-foundation.org, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, mhiramat@kernel.org, rppt@kernel.org, hughd@google.com, pcc@google.com, steven.price@arm.com, vincenzo.frascino@arm.com, david@redhat.com, eugenis@google.com, kcc@google.com, hyesoo.yu@samsung.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH RFC v3 09/35] mm: cma: Introduce cma_remove_mem() Message-ID: References: <20240125164256.4147-1-alexandru.elisei@arm.com> <20240125164256.4147-10-alexandru.elisei@arm.com> <830691cf-cb96-443e-b6eb-2adfe2edd587@arm.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <830691cf-cb96-443e-b6eb-2adfe2edd587@arm.com> Hi, I really appreciate the feedback you have given me so far. I believe the commit message isn't clear enough and there has been a confusion. A CMA user adds a CMA area to the cma_areas array with cma_declare_contiguous_nid() or cma_init_reserved_mem(). init_cma_reserved_pageblock() then iterates over the array and activates all cma areas. The function cma_remove_mem() is intended to be used to remove a cma area from the cma_areas array **before** the area has been activated. Usecase: a driver (in this case, the arm64 dynamic tag storage code) manages several cma areas. The driver successfully adds the first area to the cma_areas array. When the driver tries to adds the second area, the function fails. Without cma_remove_mem(), the driver has no way to prevent the first area from being freed to the page allocator. cma_remove_mem() is about providing a means to do cleanup in case of error. Does that make more sense now? Ok Tue, Jan 30, 2024 at 11:20:56AM +0530, Anshuman Khandual wrote: > > > On 1/25/24 22:12, Alexandru Elisei wrote: > > Memory is added to CMA with cma_declare_contiguous_nid() and > > cma_init_reserved_mem(). This memory is then put on the MIGRATE_CMA list in > > cma_init_reserved_areas(), where the page allocator can make use of it. > > cma_declare_contiguous_nid() reserves memory in memblock and marks the You forgot about about cma_init_reserved_mem() which does the same thing, but yes, you are right. > for subsequent CMA usage, where as cma_init_reserved_areas() activates > these memory areas through init_cma_reserved_pageblock(). Standard page > allocator only receives these memory via free_reserved_page() - only if I don't think that's correct. init_cma_reserved_pageblock() clears the PG_reserved page flag, sets the migratetype to MIGRATE_CMA and then frees the page. After that, the page is available to the standard page allocator to use for allocation. Otherwise, what would be the point of the MIGRATE_CMA migratetype? > the page block activation fails. For the sake of having a complete picture, I'll add that that only happens if cma->reserve_pages_on_error is false. If the CMA user sets the field to 'true' (with cma_reserve_pages_on_error()), then the pages in the CMA region are kept PG_reserved if activation fails. > > > > > If a device manages multiple CMA areas, and there's an error when one of > > the areas is added to CMA, there is no mechanism for the device to prevent > > What kind of error ? init_cma_reserved_pageblock() fails ? But that will > not happen until cma_init_reserved_areas(). I think I haven't been clear enough. When I say that "an area is added to CMA", I mean that the memory region is added to cma_areas array, via cma_declare_contiguous_nid() or cma_init_reserved_mem(). There are several ways in which either function can fail. > > > the rest of the areas, which were added before the error occured, from > > being later added to the MIGRATE_CMA list. > > Why is this mechanism required ? cma_init_reserved_areas() scans over all > CMA areas and try and activate each of them sequentially. Why is not this > sufficient ? This patch is about removing a struct cma from the cma_areas array after it has been added to the array, with cma_declare_contiguous_nid() or cma_init_reserved_mem(), to prevent the area from being activated in cma_init_reserved_areas(). Sorry for the confusion. I'll add a check in cma_remove_mem() to fail if the cma area has been activated, and a comment to the function to explain its usage. > > > > > Add cma_remove_mem() which allows a previously reserved CMA area to be > > removed and thus it cannot be used by the page allocator. > > Successfully activated CMA areas do not get used by the buddy allocator. I don't believe that is correct, see above. > > > > > Signed-off-by: Alexandru Elisei > > --- > > > > Changes since rfc v2: > > > > * New patch. > > > > include/linux/cma.h | 1 + > > mm/cma.c | 30 +++++++++++++++++++++++++++++- > > 2 files changed, 30 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/cma.h b/include/linux/cma.h > > index e32559da6942..787cbec1702e 100644 > > --- a/include/linux/cma.h > > +++ b/include/linux/cma.h > > @@ -48,6 +48,7 @@ extern int cma_init_reserved_mem(phys_addr_t base, phys_addr_t size, > > unsigned int order_per_bit, > > const char *name, > > struct cma **res_cma); > > +extern void cma_remove_mem(struct cma **res_cma); > > extern struct page *cma_alloc(struct cma *cma, unsigned long count, unsigned int align, > > bool no_warn); > > extern int cma_alloc_range(struct cma *cma, unsigned long start, unsigned long count, > > diff --git a/mm/cma.c b/mm/cma.c > > index 4a0f68b9443b..2881bab12b01 100644 > > --- a/mm/cma.c > > +++ b/mm/cma.c > > @@ -147,8 +147,12 @@ static int __init cma_init_reserved_areas(void) > > { > > int i; > > > > - for (i = 0; i < cma_area_count; i++) > > + for (i = 0; i < cma_area_count; i++) { > > + /* Region was removed. */ > > + if (!cma_areas[i].count) > > + continue; > > Skip previously added CMA area (now zeroed out) ? Yes, that's what I meant with the comment "Region was removed". Do you think I should reword the comment? > > > cma_activate_area(&cma_areas[i]); > > + } > > > > return 0; > > } > > cma_init_reserved_areas() gets called via core_initcall(). Some how > platform/device needs to call cma_remove_mem() before core_initcall() > gets called ? This might be time sensitive. I don't understand your point. > > > @@ -216,6 +220,30 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size, > > return 0; > > } > > > > +/** > > + * cma_remove_mem() - remove cma area > > + * @res_cma: Pointer to the cma region. > > + * > > + * This function removes a cma region created with cma_init_reserved_mem(). The > > + * ->count is set to 0. > > + */ > > +void __init cma_remove_mem(struct cma **res_cma) > > +{ > > + struct cma *cma; > > + > > + if (WARN_ON_ONCE(!res_cma || !(*res_cma))) > > + return; > > + > > + cma = *res_cma; > > + if (WARN_ON_ONCE(!cma->count)) > > + return; > > + > > + totalcma_pages -= cma->count; > > + cma->count = 0; > > + > > + *res_cma = NULL; > > +} > > + > > /** > > * cma_declare_contiguous_nid() - reserve custom contiguous area > > * @base: Base address of the reserved area optional, use 0 for any > > But first please do explain what are the errors device or platform might cma_declare_contiguous_nid() and cma_init_reserved_mem() can fail in a number of ways, the code should be self documenting. > see on a previously marked CMA area so that removing them on way becomes > necessary preventing their activation via cma_init_reserved_areas(). I've described how the function is supposed to be used at the top of my reply. Thanks, Alex