mmap is used to map file(devices) or shared memory to process virtual address.
So, for any device or file, which you want to map, everything is treated as file. So, in all cases you are trying to avoid file system. Typically read(),write() all these functions are generic functions provided by filesystem for a uniform access to all devices and files hiding the details.
So, when you are using mmap, you are excluding file system and directly accessing memory as if it is within process address space.
Typically when you map certain virtual address of process, depending on requested memory, those size pages are allocated to the calling process.
Now depending upon what you have mapped to, the OS is going to perform operations on that entity for the memory addressed in store and load instructions. This entity can be device, file stored on IDE, shared memory segment across networks. So, this also an abstraction provided by OS. ok.
Regards,
Sri.
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Regards,
On Mon, Sep 20, 2010 at 1:33 AM, Rajat Jain <rajatjain@xxxxxxxxxxx> wrote:
Hello List,
I wanted to understand how the mmap works. I mean, not from the kernel APIs or data structures perspective, but from the HW perspective. I understand that the use case is this: typically applications will mmap() a file (a device file or any other file) into user space, and then use direct pointers to read / write to that file. So as I understand, after the call to mmap, the compiler will generate simple load and store instructions fo any reads / writes into that file. What I'm trying to understand is that how is it ensured that those load & store instructions get translated into file reads/ writes. I think there are two cases:
1) If the backing file is /dev/mem or a device file, then at the time of mmap(), the page table entries / TLB entries are set up such that the virtual address in the user space, maps to the actual physical address desired. And the thus job is done. This case is clear.
2) The other case is that when the file is actually a datafile present in a file system lets say on a SATA disk. Now, for each load / store in the application, some driver code needs to trigger that will program the SATA controller registers and do n number of thinks to initiate the diska read / write transaction. How is this achieved? Because application is not making any System calls. Its just having load / store instructions.
Thanks,
Rajat Jain
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Regards,
Sri.
