On 2024/11/8 17:04, Matteo Martelli wrote: > On Mon, 28 Oct 2024 13:04:10 +0100, matteomartelli3@xxxxxxxxx wrote: >> Hi everyone, >> >> I found an issue that might interest iio, sysfs and devres, about a >> particular usage of devm_kmalloc() for buffers that later pass through >> sysfs_emit() or sysfs_emit_at(). These sysfs helpers require the output >> buffer to be PAGE_SIZE aligned since commit 2efc459d06f1 ("sysfs: Add >> sysfs_emit and sysfs_emit_at to format sysfs output"). Such requirement >> is satisfied when kmalloc(PAGE_SIZE, ...) is used but not when >> devm_kmalloc(PAGE_SIZE,...) is used as it actually returns a pointer to >> a buffer located after the devres metadata and thus aligned to >> PAGE_SIZE+sizeof(struct devres). >> >> Specifically, I came across this issue during some testing of the >> pac1921 iio driver together with the iio-mux iio consumer driver, which >> allocates a page sized buffer to copy the ext_info of the producer >> pac1921 iio producer driver. To fill the buffer, the latter calls >> iio_format_value(), and so sysfs_emit_at() which fails due to the buffer >> not being page aligned. This pattern seems common for many iio drivers >> which fill the ext_info attributes through sysfs_emit*() helpers, likely >> necessary as they are exposed on sysfs. >> >> I could reproduce the same error behavior with a minimal dummy char >> device driver completely unrelated to iio. I will share the entire dummy >> driver code if needed but essentially this is the only interesting part: >> >> data->info_buf = devm_kzalloc(data->dev, PAGE_SIZE, GFP_KERNEL); >> if (!data->info_buf) >> return -ENOMEM; >> >> if (offset_in_page(data->info_buf)) >> pr_err("dummy_test: buf not page algined\n"); >> >> When running this, the error message is printed out for the reason above. >> >> I am not sure whether this should be addressed in the users of >> devm_kmalloc() or in the devres implementation itself. I would say that >> it would be more clear if devm_kmalloc() would return the pointer to the >> size aligned buffer, as it would also comply to the following kmalloc >> requirement (introduced in [1]): >> >> The address of a chunk allocated with `kmalloc` is aligned to at least >> ARCH_KMALLOC_MINALIGN bytes. For sizes of power of two bytes, the >> alignment is also guaranteed to be at least to the respective size. >> >> To do so I was thinking to try to move the devres metadata after the >> data buffer, so that the latter would directly correspond to pointer >> returned by kmalloc. I then found out that it had been already suggested >> previously to address a memory optimization [2]. Thus I am reporting the >> issue before submitting any patch as some discussions might be helpful >> first. >> no, IMO, that is not good idea absolutely. >> I am sending this to who I think might be interested based on previous >> related activity. Feel free to extend the cc list if needed. > > Adding some more context to better understand the impact of this. > > With a trivial grep it looks like there are only few instances where > devm_k*alloc() is used to allocate a PAGE_SIZE buffer: > > $ git grep -n 'devm_.*alloc.*(.*PAGE_SIZE' > block/badblocks.c:1584: bb->page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL); > drivers/iio/multiplexer/iio-mux.c:287: page = devm_kzalloc(dev, PAGE_SIZE, GFP_KERNEL); > drivers/mtd/nand/raw/mxc_nand.c:1702: host->data_buf = devm_kzalloc(&pdev->dev, PAGE_SIZE, GFP_KERNEL); > drivers/usb/gadget/udc/gr_udc.c:1987: buf = devm_kzalloc(dev->dev, PAGE_SIZE, GFP_DMA | GFP_ATOMIC); > sound/soc/sof/debug.c:277: dfse->buf = devm_kmalloc(sdev->dev, PAGE_SIZE, GFP_KERNEL); > > What takes my attention is the bb->page in blocks/badblocks.c, being the > buffer named "page" maybe it is supposed to be page aligned? > > Also in [3] it was suggested to add the page alignment check for > sysfs_emit() and sysfs_emit_at(), but I haven't found why that's > necessary. My guess is for optimizations to avoid the buffer to spread > in more than one page. Is this correct? Are there other reasons? Can > anyone add more details? I think it would help to understand whether > page alignment is necessary in the other instances of devm_k*alloc(). > > Beside page alignment, there are plenty of devm_k*alloc() around the > code base, is there any way to spot whether any of those instances > expect the allocated buffer to be aligned to the provided size? > > If this is a limited use-case it can be worked around with just regular > k*alloc() + devm_add_action_or_reset() as Jonathan suggested. However, I > still think it can be easy to introduce some alignment related bug, > especially when transitioning from k*alloc() to devm_k*alloc() in an old > implementation since it can be assumed that they have the same alignment > guarantees. Maybe some comment in the devres APIs or documentation would > help in this case? > > Any thoughts? > why not to use existing APIs? addr = devm_get_free_pages(dev, GFP_KERNEL|__GFP_ZERO, 0); devm_free_pages(dev,addr); >> >> [1]: https://lore.kernel.org/all/20190826111627.7505-3-vbabka@xxxxxxx/ >> [2]: https://lore.kernel.org/all/20191220140655.GN2827@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > [3]: https://lore.kernel.org/all/743a648dc817cddd2e7046283c868f1c08742f29.camel@xxxxxxxxxxx/ > > Best regards, > Matteo Martelli >