From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Fainelli Subject: Re: DMA allocations from CMA and fatal_signal_pending check Date: Mon, 03 Nov 2014 10:51:32 -0800 Message-ID: <5457CEB4.9020700@gmail.com> References: <544FE9BE.6040503@gmail.com> <20141031082818.GB14642@js1304-P5Q-DELUXE> <5453F80C.4090006@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Cc: linux-arm-kernel@lists.infradead.org, Brian Norris , Gregory Fong , linux-kernel@vger.kernel.org, linux-mm@kvack.org, lauraa@codeaurora.org, gioh.kim@lge.com, aneesh.kumar@linux.vnet.ibm.com, m.szyprowski@samsung.com, akpm@linux-foundation.org, "netdev@vger.kernel.org" To: Michal Nazarewicz , Joonsoo Kim Return-path: In-Reply-To: Sender: owner-linux-mm@kvack.org List-Id: netdev.vger.kernel.org On 11/03/2014 08:45 AM, Michal Nazarewicz wrote: > On Fri, Oct 31 2014, Florian Fainelli wrote: >> I agree that the CMA allocation should not be allowed to succeed, but >> the dma_alloc_coherent() allocation should succeed. If we look at the >> sysport driver, there are kmalloc() calls to initialize private >> structures, those will succeed (except under high memory pressure), so >> by the same token, a driver expects DMA allocations to succeed (unless >> we are under high memory pressure) >> >> What are we trying to solve exactly with the fatal_signal_pending() >> check here? Are we just optimizing for the case where a process has >> allocated from a CMA region to allow this region to be returned to the >> pool of free pages when it gets killed? Could there be another mechani= sm >> used to reclaim those pages if we know the process is getting killed >> anyway? >=20 > We're guarding against situations where process may hang around > arbitrarily long time after receiving SIGKILL. If user does =E2=80=9Ck= ill -9 > $pid=E2=80=9D the usual expectation is that the $pid process will die w= ithin > seconds and anything longer is perceived by user as a bug. >=20 > What problem are *you* trying to solve? If user sent SIGKILL to > a process that imitated device initialisation, what is the point of > continuing initialising the device? Just recover and return -EINTR. I have two problems with the current approach: - behavior of a dma_alloc_coherent() call is not consistent between a CONFIG_CMA=3Dy vs. CONFIG_CMA=3Dn build, which is probably fine as long a= s we document that properly - there is currently no way for a caller of dma_alloc_coherent to tell whether the allocation failed because it was interrupted by a signal, a genuine OOM or something else, this is largely made worse by problem 1 >=20 >> Well, not really. This driver is not an isolated case, there are tons = of >> other networking drivers that do exactly the same thing, and we do >> expect these dma_alloc_* calls to succeed. >=20 > Again, why do you expect them to succeed? The code must handle failure= s > correctly anyway so why do you wish to ignore fatal signal? I guess expecting them to succeed is probably not good, but at we should at least be able to report an accurate error code to the caller and down to user-space. Thanks -- Florian -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org