src/lib/file-cache.c

/* Copyright (c) 2004-2018 Dovecot authors, see the included COPYING file */

#include "lib.h"
#include "buffer.h"
#include "mmap-util.h"
#include "file-cache.h"

#include <sys/stat.h>

struct file_cache {
    int fd;
    char *path;
    buffer_t *page_bitmask;

    void *mmap_base;
    size_t mmap_length;
    size_t read_highwater;
};

struct file_cache *file_cache_new(int fd)
{
    return file_cache_new_path(fd, "");
}

struct file_cache *file_cache_new_path(int fd, const char *path)
{
    struct file_cache *cache;

    cache = i_new(struct file_cache, 1);
    cache->fd = fd;
    cache->path = i_strdup(path);
    cache->page_bitmask = buffer_create_dynamic(default_pool, 128);
    return cache;
}

void file_cache_free(struct file_cache **_cache)
{
    struct file_cache *cache = *_cache;

    *_cache = NULL;

    if (cache->mmap_base != NULL) {
        if (munmap_anon(cache->mmap_base, cache->mmap_length) < 0)
            i_error("munmap_anon(%s) failed: %m", cache->path);
    }
    buffer_free(&cache->page_bitmask);
    i_free(cache->path);
    i_free(cache);
}

void file_cache_set_fd(struct file_cache *cache, int fd)
{
    cache->fd = fd;
    file_cache_invalidate(cache, 0, cache->mmap_length);
}

int file_cache_set_size(struct file_cache *cache, uoff_t size)
{
    size_t page_size = mmap_get_page_size();
    uoff_t diff;
    void *new_base;

    i_assert(page_size > 0);

    diff = size % page_size;
    if (diff != 0)
        size += page_size - diff;

    i_assert((size % page_size) == 0);
    if (size <= cache->mmap_length)
        return 0;

    if (size > (size_t)-1) {
        i_error("file_cache_set_size(%s, %"PRIuUOFF_T"): size too large",
            cache->path, size);
        return -1;
    }

    /* grow mmaping */
    if (cache->mmap_base == NULL) {
        cache->mmap_base = mmap_anon(size);
        if (cache->mmap_base == MAP_FAILED) {
            i_error("mmap_anon(%s, %"PRIuUOFF_T") failed: %m",
                cache->path, size);
            cache->mmap_base = NULL;
            cache->mmap_length = 0;
            return -1;
        }
    } else {
        new_base = mremap_anon(cache->mmap_base, cache->mmap_length,
                       size, MREMAP_MAYMOVE);
        if (new_base == MAP_FAILED) {
            i_error("mremap_anon(%s, %"PRIuUOFF_T") failed: %m",
                cache->path, size);
            return -1;
        }

        cache->mmap_base = new_base;
    }
    cache->mmap_length = size;
    return 0;
}

ssize_t file_cache_read(struct file_cache *cache, uoff_t offset, size_t size)
{
    size_t page_size = mmap_get_page_size();
    size_t poffset, psize, dest_offset, dest_size;
    unsigned char *bits, *dest;
    ssize_t ret;

    i_assert(page_size > 0);

    if (size > SSIZE_T_MAX) {
        /* make sure our calculations won't overflow. most likely
           we'll be reading less data, but allow it anyway so caller
           doesn't have to deal with any extra checks. */
        size = SSIZE_T_MAX;
    }
    if (offset >= (uoff_t)-1 - size)
        size = (uoff_t)-1 - offset;

    if (offset + size > cache->mmap_length &&
        offset + size - cache->mmap_length > 1024*1024) {
        /* growing more than a megabyte, make sure that the
           file is large enough so we don't allocate memory
           more than needed */
        struct stat st;

                if (fstat(cache->fd, &st) < 0) {
                        if (errno != ESTALE)
                                i_error("fstat(%s) failed: %m", cache->path);
            return -1;
        }

        if (offset + size > (uoff_t)st.st_size) {
            if (offset >= (uoff_t)st.st_size)
                return 0;
            size = (uoff_t)st.st_size - offset;
        }
    }

    if (file_cache_set_size(cache, offset + size) < 0)
        return -1;

    poffset = offset / page_size;
    psize = (offset + size + page_size-1) / page_size - poffset;
    i_assert(psize > 0);

    bits = buffer_get_space_unsafe(cache->page_bitmask, 0,
                       (poffset + psize + CHAR_BIT - 1) /
                       CHAR_BIT);

    dest_offset = poffset * page_size;
    dest = PTR_OFFSET(cache->mmap_base, dest_offset);
    dest_size = page_size;

    while (psize > 0) {
        if ((bits[poffset / CHAR_BIT] & (1 << (poffset % CHAR_BIT))) != 0) {
            /* page is already in cache */
            dest_offset += page_size;
            if (dest_offset <= cache->read_highwater) {
                psize--; poffset++;
                dest += page_size;
                continue;
            }

            /* this is the last partially cached block.
               use the caching only if we don't want to
               read past read_highwater */
            if (offset + size <= cache->read_highwater) {
                i_assert(psize == 1);
                break;
            }

            /* mark the block noncached again and
               read it */
            bits[poffset / CHAR_BIT] &=
                ~(1 << (poffset % CHAR_BIT));
            dest_offset -= page_size;
        }

        ret = pread(cache->fd, dest, dest_size, dest_offset);
        if (ret <= 0) {
            if (ret < 0)
                return -1;

            /* EOF. mark the last block as cached even if it
               isn't completely. read_highwater tells us how far
               we've actually made. */
            if (dest_offset == cache->read_highwater) {
                i_assert(poffset ==
                     cache->read_highwater / page_size);
                bits[poffset / CHAR_BIT] |=
                    1 << (poffset % CHAR_BIT);
            }
            return dest_offset <= offset ? 0 :
                dest_offset - offset < size ?
                dest_offset - offset : size;
        }

        dest += ret;
        dest_offset += ret;

        if (cache->read_highwater < dest_offset) {
            unsigned int high_poffset =
                cache->read_highwater / page_size;

            /* read_highwater needs to be updated. if we didn't
               just read that block, we can't trust anymore that
               we have it cached */
            bits[high_poffset / CHAR_BIT] &=
                ~(1 << (high_poffset % CHAR_BIT));
            cache->read_highwater = dest_offset;
        }

        if ((size_t)ret != dest_size) {
            /* partial read - probably EOF but make sure. */
            dest_size -= ret;
            continue;
        }

        bits[poffset / CHAR_BIT] |= 1 << (poffset % CHAR_BIT);
        dest_size = page_size;
        psize--; poffset++;
    }

    return size;
}

const void *file_cache_get_map(struct file_cache *cache, size_t *size_r)
{
    *size_r = cache->read_highwater;
    return cache->mmap_base;
}

void file_cache_write(struct file_cache *cache, const void *data, size_t size,
              uoff_t offset)
{
    size_t page_size = mmap_get_page_size();
    unsigned char *bits;
    unsigned int first_page, last_page;

    i_assert(page_size > 0);
    i_assert((uoff_t)-1 - offset > size);

    if (file_cache_set_size(cache, offset + size) < 0) {
        /* couldn't grow mapping. just make sure the written memory
           area is invalidated then. */
        file_cache_invalidate(cache, offset, size);
        return;
    }

    memcpy(PTR_OFFSET(cache->mmap_base, offset), data, size);

    if (cache->read_highwater < offset + size) {
        unsigned int page = cache->read_highwater / page_size;

        bits = buffer_get_space_unsafe(cache->page_bitmask,
                           page / CHAR_BIT, 1);
        *bits &= ~(1 << (page % CHAR_BIT));
        cache->read_highwater = offset + size;
    }

    /* mark fully written pages cached */
    if (size >= page_size) {
        first_page = offset / page_size;
        last_page = (offset + size) / page_size;
        if ((offset % page_size) != 0)
            first_page++;

        bits = buffer_get_space_unsafe(cache->page_bitmask, 0,
                           last_page / CHAR_BIT + 1);
        for (; first_page < last_page; first_page++) {
            bits[first_page / CHAR_BIT] |=
                1 << (first_page % CHAR_BIT);
        }
    }
}

void file_cache_invalidate(struct file_cache *cache, uoff_t offset, uoff_t size)
{
    size_t page_size = mmap_get_page_size();
    unsigned char *bits, mask;
    unsigned int i;

    if (offset >= cache->read_highwater || size == 0)
        return;

    i_assert(page_size > 0);

    if (size > cache->read_highwater - offset) {
        /* ignore anything after read highwater */
        size = cache->read_highwater - offset;
    }
    if (size >= cache->read_highwater) {
        /* we're invalidating everything up to read highwater.
           drop the highwater position. */
        cache->read_highwater = offset & ~(page_size-1);
    }

    size = (offset + size + page_size-1) / page_size;
    offset /= page_size;
    i_assert(size > offset);
    size -= offset;

    if (size != 1) {
        /* tell operating system that we don't need the memory anymore
           and it may free it. don't bother to do it for single pages,
           there's a good chance that they get re-read back
           immediately. */
        (void)madvise(PTR_OFFSET(cache->mmap_base, offset * page_size),
                  size * page_size, MADV_DONTNEED);
    }

    bits = buffer_get_space_unsafe(cache->page_bitmask, offset / CHAR_BIT,
                       1 + (size + CHAR_BIT - 1) / CHAR_BIT);

    /* set the first byte */
    for (i = offset % CHAR_BIT, mask = 0; i < CHAR_BIT && size > 0; i++) {
        mask |= 1 << i;
        size--;
    }
    *bits++ &= ~mask;

    /* set the middle bytes */
    memset(bits, 0, size / CHAR_BIT);
    bits += size / CHAR_BIT;
    size %= CHAR_BIT;

    /* set the last byte */
    if (size > 0) {
        for (i = 0, mask = 0; i < size; i++)
            mask |= 1 << i;
        *bits &= ~mask;
    }
}