From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lukasz Majewski Date: Tue, 26 Jan 2016 09:54:02 +0100 Subject: [U-Boot] [PATCH] malloc: work around some memalign fragmentation issues In-Reply-To: <1453755822-28569-1-git-send-email-swarren@wwwdotorg.org> References: <1453755822-28569-1-git-send-email-swarren@wwwdotorg.org> Message-ID: <20160126095402.1c80368a@amdc2363> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de Hi Stephen, > From: Stephen Warren > > Use of memalign can trigger fragmentation issues such as: > > // Internally, this needs to find a free block quite bit larger than > s. // Once the free region is found, any unaligned "padding" > immediately // before and after the block is marked free, so that the > allocation // takes only s bytes (plus malloc header overhead). > p = memalign(a, s); > // If there's little fragmentation so far, this allocation is likely > // located immediately after p. > p2 = malloc(x); > free(p); > // In theory, this should return the same value for p. However, the > hole // left by the free() call is only s in size (plus malloc header > overhead) // whereas memalign searches for a larger block in order to > guarantee it // can adjust the returned pointer to the alignment > requirements. Hence, // the pointer returned, if any, won't be p. If > there's little or no space // left after p2, this allocation will > fail. p = memalign(a, s); > > In practice, this issue occurs when running the "dfu" command > repeatedly on NVIDIA Tegra boards, since DFU allocates a large 32M > data buffer, and then initializes the USB controller. If this is the > first time USB has been used in the U-Boot session, this causes a > probe of the USB driver, which causes various allocations, including > a strdup() of a GPIO name when requesting the VBUS GPIO. When DFU is > torn down, the USB driver is left probed, and hence its memory is > left allocated. If "dfu" is executed again, allocation of the 32M > data buffer fails as described above. > > In practice, there is a memory hole exactly large enough to hold the > 32M data buffer than DFU needs. However, memalign() can't know that > in a general way. Given that, it's particularly annoying that the > allocation fails! > > The issue is that memalign() tries to allocate something larger to > guarantee the ability to align the returned pointer. This patch > modifies memalign() so that if the "general case" over-sized > allocation fails, another allocation is attempted, of the exact size > the user desired. If that allocation just happens to be aligned in > the way the user wants, (and in the case described above, it will be, > since the free memory region is located where a previous identical > allocation was located), the pointer can be returned. > > This patch is somewhat related to 806bd245b1ab "dfu: don't keep > freeing/reallocating". That patch worked around the issue by removing > repeated free/memalign within a single execution of "dfu". However, > the same technique can't be applied across multiple invocations, since > there's no reason to keep the DFU buffer allocated while DFU isn't > running. This patch addresses the root-cause a bit more directly. > > This problem highlights some of the disadvantages of dynamic > allocation and deferred probing of devices. > > This patch isn't checkpatch-clean, since it conforms to the existing > coding style in dlmalloc.c, which is different to the rest of U-Boot. > > Signed-off-by: Stephen Warren > --- > common/dlmalloc.c | 22 ++++++++++++++++++++++ > 1 file changed, 22 insertions(+) > > diff --git a/common/dlmalloc.c b/common/dlmalloc.c > index b5bb05191c24..2b964d16b11e 100644 > --- a/common/dlmalloc.c > +++ b/common/dlmalloc.c > @@ -2829,6 +2829,28 @@ Void_t* mEMALIGn(alignment, bytes) size_t > alignment; size_t bytes; nb = request2size(bytes); > m = (char*)(mALLOc(nb + alignment + MINSIZE)); > > + /* > + * The attempt to over-allocate (with a size large enough to > guarantee the > + * ability to find an aligned region within allocated memory) > failed. > + * > + * Try again, this time only allocating exactly the size the user > wants. If > + * the allocation now succeeds and just happens to be aligned, we > can still > + * fulfill the user's request. > + */ > + if (m == NULL) { > + /* > + * Use bytes not nb, since mALLOc internally calls request2size > too, and > + * each call increases the size to allocate, to account for the > header. > + */ > + m = (char*)(mALLOc(bytes)); > + /* Aligned -> return it */ > + if ((((unsigned long)(m)) % alignment) == 0) > + return m; > + /* Otherwise, fail */ > + fREe(m); > + return NULL; > + } > + > if (m == NULL) return NULL; /* propagate failure */ > > p = mem2chunk(m); U-boot's ./common/dlmalloc.c file is from year 2000 (version 2.6.6). I'm just wondering if there exists newer version of this code (and can be easily ported to u-boot). Despite the above note, Acked-by: Lukasz Majewski -- Best regards, Lukasz Majewski Samsung R&D Institute Poland (SRPOL) | Linux Platform Group