Messages in this thread | | | Subject | Re: [PATCH v7 1/8] fs: introduce kernel_pread_file* support | From | Scott Branden <> | Date | Tue, 9 Jun 2020 15:55:15 -0700 |
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Hi Matthew,
On 2020-06-09 6:21 a.m., Matthew Wilcox wrote: > On Mon, Jun 08, 2020 at 03:29:22PM -0700, Scott Branden wrote: >> Hi Matthew, >> >> I am requesting the experts in the filesystem subsystem to come to a >> consensus here. >> This is not my area of expertise at all but every time I have addressed all >> of the >> outstanding concerns someone else comes along and raises another one. > I appreciate it's frustrating for you, but this is the nature of > patch review. I haven't even read the first five or so submissions. > I can see them in my inbox and they look like long threads. I'm not > particularly inclined to read them. I happened to read v6, and reacted > to the API being ugly. Thanks for the review. Yes, I do see the enum being ugly now and have removed it in v8 of the patch. Hopefully it addresses your concerns. More comments below. > >> Please see me comments below. >> >> On 2020-06-06 8:52 a.m., Matthew Wilcox wrote: >>> On Fri, Jun 05, 2020 at 10:04:51PM -0700, Scott Branden wrote: >>>> -int kernel_read_file(struct file *file, void **buf, loff_t *size, >>>> - loff_t max_size, enum kernel_read_file_id id) >>>> -{ >>>> - loff_t i_size, pos; >> Please note that how checkpatch generated the diff here. The code >> modifications >> below are for a new function kernel_pread_file, they do not modify the >> existing API >> kernel_read_file. kernel_read_file requests the ENTIRE file is read. So we >> need to be >> able to differentiate whether it is ok to read just a portion of the file or >> not. > You've gone about this in entirely the wrong way though. This enum to > read the entire file or a partial is just bad design. Your point on the enum is valid. I've removed it from design. Hopefully it is cleaner now. > >>>> +int kernel_pread_file(struct file *file, void **buf, loff_t *size, >>>> + loff_t pos, loff_t max_size, >>>> + enum kernel_pread_opt opt, >>>> + enum kernel_read_file_id id) >> So, to share common code a new kernel_pread_opt needed to be added in order >> to specify whether >> it was ok to read a partial file or not, and provide an offset into the file >> where to begin reading. >> The meaning of parameters doesn't change in the bonkers API. max_size still >> means max size, etc. >> These options are needed so common code can be shared with kernel_read_file >> api. > Does pread() in userspace take seven parameters? No. It takes four. > What you're doing is taking all the complexity of all of the interfaces > and stuffing it all down into the bottom function instead of handling > some of the complexity in the wrapper functions. For example, you > could support the functionality of 'max_size' in kernel_read_file() > and leave it out of the kernel_pread_file() interface. I have removed the enum necessary in the kernel pread call now, so it is down to 6. The other 2 parameters are necessary as they are in kernel read.
max_size makes no sense to remove - it serves the same purpose as in userspace pread and read functions. To specify the max size to read. >>> I think what we actually want is: >>> >>> ssize_t vmap_file_range(struct file *, loff_t start, loff_t end, void **bufp); >>> void vunmap_file_range(struct file *, void *buf); >>> >>> If end > i_size, limit the allocation to i_size. Returns the number >>> of bytes allocated, or a negative errno. Writes the pointer allocated >>> to *bufp. Internally, it should use the page cache to read in the pages >>> (taking appropriate reference counts). Then it maps them using vmap() >>> instead of copying them to a private vmalloc() array. >>> kernel_read_file() can be converted to use this API. The users will >>> need to be changed to call kernel_read_end(struct file *file, void *buf) >>> instead of vfree() so it can call allow_write_access() for them. >>> >>> vmap_file_range() has a lot of potential uses. I'm surprised we don't >>> have it already, to be honest. >> Such a change sounds like it could be done in a later patch series. >> It's an incomplete solution. It would work for some of the needed >> operations but not others. >> For kernel_read_file, I don't see how in your new API it indicates if the >> end of the file was reached or not. > That's the point. It doesn't. If a caller needs that, then they can > figure that out themselves. No, they can't. The caller only calls kernel_read_file once and expects the whole file to be read. The kernel_read_file doesn't work like userspace. There is no tracking like userspace of where in the file you read? > >> Also, please note that buffers may be preallocated and shouldn't be freed >> by the kernel in some cases and >> allocated and freed by the kernel in others. > You're trying to build the swiss army knife of functions. Swiss army > knives are useful, but they're no good for carving a steak. Hopefully I'm carving steak now. >> I would like the experts here to decide on what needs to be done so we can >> move forward >> and get kernel_pread_file support added soon. > You know, you haven't even said _why_ you want this. The cover letter > just says "I want this", and doesn't say why it's needed. Cover letter updated.
Thanks, Scott
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